define the importance of literature review

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Literature Reviews

• What is a literature review?
• Steps in the Literature Review Process
• Define your research question
• Determine inclusion and exclusion criteria
• Choose databases and search
• Review Results
• Synthesize Results
• Analyze Results
• Librarian Support

What is a Literature Review?

A literature or narrative review is a comprehensive review and analysis of the published literature on a specific topic or research question. The literature that is reviewed contains: books, articles, academic articles, conference proceedings, association papers, and dissertations. It contains the most pertinent studies and points to important past and current research and practices. It provides background and context, and shows how your research will contribute to the field. 

A literature review should: 

• Provide a comprehensive and updated review of the literature;
• Explain why this review has taken place;
• Articulate a position or hypothesis;
• Acknowledge and account for conflicting and corroborating points of view

From  S age Research Methods

Purpose of a Literature Review

A literature review can be written as an introduction to a study to:

• Demonstrate how a study fills a gap in research
• Compare a study with other research that's been done

Or it can be a separate work (a research article on its own) which:

• Organizes or describes a topic
• Describes variables within a particular issue/problem

Limitations of a Literature Review

Some of the limitations of a literature review are:

• It's a snapshot in time. Unlike other reviews, this one has beginning, a middle and an end. There may be future developments that could make your work less relevant.
• It may be too focused. Some niche studies may miss the bigger picture.
• It can be difficult to be comprehensive. There is no way to make sure all the literature on a topic was considered.
• It is easy to be biased if you stick to top tier journals. There may be other places where people are publishing exemplary research. Look to open access publications and conferences to reflect a more inclusive collection. Also, make sure to include opposing views (and not just supporting evidence).

Source: Grant, Maria J., and Andrew Booth. “A Typology of Reviews: An Analysis of 14 Review Types and Associated Methodologies.” Health Information & Libraries Journal, vol. 26, no. 2, June 2009, pp. 91–108. Wiley Online Library, doi:10.1111/j.1471-1842.2009.00848.x.

Meryl Brodsky : Communication and Information Studies

Hannah Chapman Tripp : Biology, Neuroscience

Carolyn Cunningham : Human Development & Family Sciences, Psychology, Sociology

Larayne Dallas : Engineering

Janelle Hedstrom : Special Education, Curriculum & Instruction, Ed Leadership & Policy ​

Susan Macicak : Linguistics

Imelda Vetter : Dell Medical School

For help in other subject areas, please see the guide to library specialists by subject .

Periodically, UT Libraries runs a workshop covering the basics and library support for literature reviews. While we try to offer these once per academic year, we find providing the recording to be helpful to community members who have missed the session. Following is the most recent recording of the workshop, Conducting a Literature Review. To view the recording, a UT login is required.

• October 26, 2022 recording
• Last Updated: Oct 26, 2022 2:49 PM
• URL: https://guides.lib.utexas.edu/literaturereviews

A Guide to Literature Reviews

Importance of a good literature review.

• Conducting the Literature Review
• Structure and Writing Style
• Types of Literature Reviews
• Citation Management Software This link opens in a new window
• Acknowledgements

A literature review is not only a summary of key sources, but  has an organizational pattern which combines both summary and synthesis, often within specific conceptual categories . A summary is a recap of the important information of the source, but a synthesis is a re-organization, or a reshuffling, of that information in a way that informs how you are planning to investigate a research problem. The analytical features of a literature review might:

• Give a new interpretation of old material or combine new with old interpretations,
• Trace the intellectual progression of the field, including major debates,
• Depending on the situation, evaluate the sources and advise the reader on the most pertinent or relevant research, or
• Usually in the conclusion of a literature review, identify where gaps exist in how a problem has been researched to date.

The purpose of a literature review is to:

• Place each work in the context of its contribution to understanding the research problem being studied.
• Describe the relationship of each work to the others under consideration.
• Identify new ways to interpret prior research.
• Reveal any gaps that exist in the literature.
• Resolve conflicts amongst seemingly contradictory previous studies.
• Identify areas of prior scholarship to prevent duplication of effort.
• Point the way in fulfilling a need for additional research.
• Locate your own research within the context of existing literature [very important].
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• Last Updated: May 10, 2024 11:34 AM
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Libraries | Research Guides

Literature reviews, what is a literature review, learning more about how to do a literature review.

• Planning the Review
• The Research Question
• Choosing Where to Search
• Organizing the Review
• Writing the Review

A literature review is a review and synthesis of existing research on a topic or research question. A literature review is meant to analyze the scholarly literature, make connections across writings and identify strengths, weaknesses, trends, and missing conversations. A literature review should address different aspects of a topic as it relates to your research question. A literature review goes beyond a description or summary of the literature you have read. 

• Sage Research Methods Core Collection This link opens in a new window SAGE Research Methods supports research at all levels by providing material to guide users through every step of the research process. SAGE Research Methods is the ultimate methods library with more than 1000 books, reference works, journal articles, and instructional videos by world-leading academics from across the social sciences, including the largest collection of qualitative methods books available online from any scholarly publisher. – Publisher

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• Last Updated: May 2, 2024 10:39 AM
• URL: https://libguides.northwestern.edu/literaturereviews

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What is the Purpose of a Literature Review?

4-minute read

• 23rd October 2023

If you’re writing a research paper or dissertation , then you’ll most likely need to include a comprehensive literature review . In this post, we’ll review the purpose of literature reviews, why they are so significant, and the specific elements to include in one. Literature reviews can:

1. Provide a foundation for current research.

2. Define key concepts and theories.

3. Demonstrate critical evaluation.

4. Show how research and methodologies have evolved.

5. Identify gaps in existing research.

6. Support your argument.

Keep reading to enter the exciting world of literature reviews!

What is a Literature Review?

A literature review is a critical summary and evaluation of the existing research (e.g., academic journal articles and books) on a specific topic. It is typically included as a separate section or chapter of a research paper or dissertation, serving as a contextual framework for a study. Literature reviews can vary in length depending on the subject and nature of the study, with most being about equal length to other sections or chapters included in the paper. Essentially, the literature review highlights previous studies in the context of your research and summarizes your insights in a structured, organized format. Next, let’s look at the overall purpose of a literature review.

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Literature reviews are considered an integral part of research across most academic subjects and fields. The primary purpose of a literature review in your study is to:

Provide a Foundation for Current Research

Since the literature review provides a comprehensive evaluation of the existing research, it serves as a solid foundation for your current study. It’s a way to contextualize your work and show how your research fits into the broader landscape of your specific area of study.  

Define Key Concepts and Theories

The literature review highlights the central theories and concepts that have arisen from previous research on your chosen topic. It gives your readers a more thorough understanding of the background of your study and why your research is particularly significant .

Demonstrate Critical Evaluation 

A comprehensive literature review shows your ability to critically analyze and evaluate a broad range of source material. And since you’re considering and acknowledging the contribution of key scholars alongside your own, it establishes your own credibility and knowledge.

Show How Research and Methodologies Have Evolved

Another purpose of literature reviews is to provide a historical perspective and demonstrate how research and methodologies have changed over time, especially as data collection methods and technology have advanced. And studying past methodologies allows you, as the researcher, to understand what did and did not work and apply that knowledge to your own research.  

Identify Gaps in Existing Research

Besides discussing current research and methodologies, the literature review should also address areas that are lacking in the existing literature. This helps further demonstrate the relevance of your own research by explaining why your study is necessary to fill the gaps.

Support Your Argument

A good literature review should provide evidence that supports your research questions and hypothesis. For example, your study may show that your research supports existing theories or builds on them in some way. Referencing previous related studies shows your work is grounded in established research and will ultimately be a contribution to the field.  

Literature Review Editing Services 

Ensure your literature review is polished and ready for submission by having it professionally proofread and edited by our expert team. Our literature review editing services will help your research stand out and make an impact. Not convinced yet? Send in your free sample today and see for yourself! 

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Conducting a literature review: why do a literature review, why do a literature review.

• How To Find "The Literature"
• Found it -- Now What?

Besides the obvious reason for students -- because it is assigned! -- a literature review helps you explore the research that has come before you, to see how your research question has (or has not) already been addressed.

You identify:

• core research in the field
• experts in the subject area
• methodology you may want to use (or avoid)
• gaps in knowledge -- or where your research would fit in

It Also Helps You:

• Publish and share your findings
• Justify requests for grants and other funding
• Identify best practices to inform practice
• Set wider context for a program evaluation
• Compile information to support community organizing

Great brief overview, from NCSU

Want To Know More?

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• Last Updated: Apr 25, 2024 1:10 PM
• URL: https://guides.lib.berkeley.edu/litreview

• UConn Library
• Literature Review: The What, Why and How-to Guide
• Introduction

Literature Review: The What, Why and How-to Guide — Introduction

• Getting Started
• How to Pick a Topic
• Strategies to Find Sources
• Evaluating Sources & Lit. Reviews
• Tips for Writing Literature Reviews
• Writing Literature Review: Useful Sites
• Citation Resources
• Other Academic Writings

What are Literature Reviews?

So, what is a literature review? "A literature review is an account of what has been published on a topic by accredited scholars and researchers. In writing the literature review, your purpose is to convey to your reader what knowledge and ideas have been established on a topic, and what their strengths and weaknesses are. As a piece of writing, the literature review must be defined by a guiding concept (e.g., your research objective, the problem or issue you are discussing, or your argumentative thesis). It is not just a descriptive list of the material available, or a set of summaries." Taylor, D.  The literature review: A few tips on conducting it . University of Toronto Health Sciences Writing Centre.

Goals of Literature Reviews

What are the goals of creating a Literature Review?  A literature could be written to accomplish different aims:

• To develop a theory or evaluate an existing theory
• To summarize the historical or existing state of a research topic
• Identify a problem in a field of research 

Baumeister, R. F., & Leary, M. R. (1997). Writing narrative literature reviews .  Review of General Psychology , 1 (3), 311-320.

What kinds of sources require a Literature Review?

• A research paper assigned in a course
• A thesis or dissertation
• A grant proposal
• An article intended for publication in a journal

All these instances require you to collect what has been written about your research topic so that you can demonstrate how your own research sheds new light on the topic.

Types of Literature Reviews

What kinds of literature reviews are written?

Narrative review: The purpose of this type of review is to describe the current state of the research on a specific topic/research and to offer a critical analysis of the literature reviewed. Studies are grouped by research/theoretical categories, and themes and trends, strengths and weakness, and gaps are identified. The review ends with a conclusion section which summarizes the findings regarding the state of the research of the specific study, the gaps identify and if applicable, explains how the author's research will address gaps identify in the review and expand the knowledge on the topic reviewed.

• Example : Predictors and Outcomes of U.S. Quality Maternity Leave: A Review and Conceptual Framework:  10.1177/08948453211037398  

Systematic review : "The authors of a systematic review use a specific procedure to search the research literature, select the studies to include in their review, and critically evaluate the studies they find." (p. 139). Nelson, L. K. (2013). Research in Communication Sciences and Disorders . Plural Publishing.

• Example : The effect of leave policies on increasing fertility: a systematic review:  10.1057/s41599-022-01270-w

Meta-analysis : "Meta-analysis is a method of reviewing research findings in a quantitative fashion by transforming the data from individual studies into what is called an effect size and then pooling and analyzing this information. The basic goal in meta-analysis is to explain why different outcomes have occurred in different studies." (p. 197). Roberts, M. C., & Ilardi, S. S. (2003). Handbook of Research Methods in Clinical Psychology . Blackwell Publishing.

• Example : Employment Instability and Fertility in Europe: A Meta-Analysis:  10.1215/00703370-9164737

Meta-synthesis : "Qualitative meta-synthesis is a type of qualitative study that uses as data the findings from other qualitative studies linked by the same or related topic." (p.312). Zimmer, L. (2006). Qualitative meta-synthesis: A question of dialoguing with texts .  Journal of Advanced Nursing , 53 (3), 311-318.

• Example : Women’s perspectives on career successes and barriers: A qualitative meta-synthesis:  10.1177/05390184221113735

Literature Reviews in the Health Sciences

• UConn Health subject guide on systematic reviews Explanation of the different review types used in health sciences literature as well as tools to help you find the right review type
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Writing a Literature Review

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A literature review is a document or section of a document that collects key sources on a topic and discusses those sources in conversation with each other (also called synthesis ). The lit review is an important genre in many disciplines, not just literature (i.e., the study of works of literature such as novels and plays). When we say “literature review” or refer to “the literature,” we are talking about the research ( scholarship ) in a given field. You will often see the terms “the research,” “the scholarship,” and “the literature” used mostly interchangeably.

Where, when, and why would I write a lit review?

There are a number of different situations where you might write a literature review, each with slightly different expectations; different disciplines, too, have field-specific expectations for what a literature review is and does. For instance, in the humanities, authors might include more overt argumentation and interpretation of source material in their literature reviews, whereas in the sciences, authors are more likely to report study designs and results in their literature reviews; these differences reflect these disciplines’ purposes and conventions in scholarship. You should always look at examples from your own discipline and talk to professors or mentors in your field to be sure you understand your discipline’s conventions, for literature reviews as well as for any other genre.

A literature review can be a part of a research paper or scholarly article, usually falling after the introduction and before the research methods sections. In these cases, the lit review just needs to cover scholarship that is important to the issue you are writing about; sometimes it will also cover key sources that informed your research methodology.

Lit reviews can also be standalone pieces, either as assignments in a class or as publications. In a class, a lit review may be assigned to help students familiarize themselves with a topic and with scholarship in their field, get an idea of the other researchers working on the topic they’re interested in, find gaps in existing research in order to propose new projects, and/or develop a theoretical framework and methodology for later research. As a publication, a lit review usually is meant to help make other scholars’ lives easier by collecting and summarizing, synthesizing, and analyzing existing research on a topic. This can be especially helpful for students or scholars getting into a new research area, or for directing an entire community of scholars toward questions that have not yet been answered.

What are the parts of a lit review?

Most lit reviews use a basic introduction-body-conclusion structure; if your lit review is part of a larger paper, the introduction and conclusion pieces may be just a few sentences while you focus most of your attention on the body. If your lit review is a standalone piece, the introduction and conclusion take up more space and give you a place to discuss your goals, research methods, and conclusions separately from where you discuss the literature itself.

Introduction:

• An introductory paragraph that explains what your working topic and thesis is
• A forecast of key topics or texts that will appear in the review
• Potentially, a description of how you found sources and how you analyzed them for inclusion and discussion in the review (more often found in published, standalone literature reviews than in lit review sections in an article or research paper)
• Summarize and synthesize: Give an overview of the main points of each source and combine them into a coherent whole
• Analyze and interpret: Don’t just paraphrase other researchers – add your own interpretations where possible, discussing the significance of findings in relation to the literature as a whole
• Critically Evaluate: Mention the strengths and weaknesses of your sources
• Write in well-structured paragraphs: Use transition words and topic sentence to draw connections, comparisons, and contrasts.

Conclusion:

• Summarize the key findings you have taken from the literature and emphasize their significance
• Connect it back to your primary research question

How should I organize my lit review?

Lit reviews can take many different organizational patterns depending on what you are trying to accomplish with the review. Here are some examples:

• Chronological : The simplest approach is to trace the development of the topic over time, which helps familiarize the audience with the topic (for instance if you are introducing something that is not commonly known in your field). If you choose this strategy, be careful to avoid simply listing and summarizing sources in order. Try to analyze the patterns, turning points, and key debates that have shaped the direction of the field. Give your interpretation of how and why certain developments occurred (as mentioned previously, this may not be appropriate in your discipline — check with a teacher or mentor if you’re unsure).
• Thematic : If you have found some recurring central themes that you will continue working with throughout your piece, you can organize your literature review into subsections that address different aspects of the topic. For example, if you are reviewing literature about women and religion, key themes can include the role of women in churches and the religious attitude towards women.
• Qualitative versus quantitative research
• Empirical versus theoretical scholarship
• Divide the research by sociological, historical, or cultural sources
• Theoretical : In many humanities articles, the literature review is the foundation for the theoretical framework. You can use it to discuss various theories, models, and definitions of key concepts. You can argue for the relevance of a specific theoretical approach or combine various theorical concepts to create a framework for your research.

What are some strategies or tips I can use while writing my lit review?

Any lit review is only as good as the research it discusses; make sure your sources are well-chosen and your research is thorough. Don’t be afraid to do more research if you discover a new thread as you’re writing. More info on the research process is available in our "Conducting Research" resources .

As you’re doing your research, create an annotated bibliography ( see our page on the this type of document ). Much of the information used in an annotated bibliography can be used also in a literature review, so you’ll be not only partially drafting your lit review as you research, but also developing your sense of the larger conversation going on among scholars, professionals, and any other stakeholders in your topic.

Usually you will need to synthesize research rather than just summarizing it. This means drawing connections between sources to create a picture of the scholarly conversation on a topic over time. Many student writers struggle to synthesize because they feel they don’t have anything to add to the scholars they are citing; here are some strategies to help you:

• It often helps to remember that the point of these kinds of syntheses is to show your readers how you understand your research, to help them read the rest of your paper.
• Writing teachers often say synthesis is like hosting a dinner party: imagine all your sources are together in a room, discussing your topic. What are they saying to each other?
• Look at the in-text citations in each paragraph. Are you citing just one source for each paragraph? This usually indicates summary only. When you have multiple sources cited in a paragraph, you are more likely to be synthesizing them (not always, but often
• Read more about synthesis here.

The most interesting literature reviews are often written as arguments (again, as mentioned at the beginning of the page, this is discipline-specific and doesn’t work for all situations). Often, the literature review is where you can establish your research as filling a particular gap or as relevant in a particular way. You have some chance to do this in your introduction in an article, but the literature review section gives a more extended opportunity to establish the conversation in the way you would like your readers to see it. You can choose the intellectual lineage you would like to be part of and whose definitions matter most to your thinking (mostly humanities-specific, but this goes for sciences as well). In addressing these points, you argue for your place in the conversation, which tends to make the lit review more compelling than a simple reporting of other sources.

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What is a Literature Review? How to Write It (with Examples)

A literature review is a critical analysis and synthesis of existing research on a particular topic. It provides an overview of the current state of knowledge, identifies gaps, and highlights key findings in the literature. 1 The purpose of a literature review is to situate your own research within the context of existing scholarship, demonstrating your understanding of the topic and showing how your work contributes to the ongoing conversation in the field. Learning how to write a literature review is a critical tool for successful research. Your ability to summarize and synthesize prior research pertaining to a certain topic demonstrates your grasp on the topic of study, and assists in the learning process. 

Table of Contents

• What is the purpose of literature review? 
• a. Habitat Loss and Species Extinction: 
• b. Range Shifts and Phenological Changes: 
• c. Ocean Acidification and Coral Reefs: 
• d. Adaptive Strategies and Conservation Efforts: 

How to write a good literature review 

• Choose a Topic and Define the Research Question: 
• Decide on the Scope of Your Review: 
• Select Databases for Searches: 
• Conduct Searches and Keep Track: 
• Review the Literature: 
• Organize and Write Your Literature Review: 
• How to write a literature review faster with Paperpal? 
• Frequently asked questions 

What is a literature review?

A well-conducted literature review demonstrates the researcher’s familiarity with the existing literature, establishes the context for their own research, and contributes to scholarly conversations on the topic. One of the purposes of a literature review is also to help researchers avoid duplicating previous work and ensure that their research is informed by and builds upon the existing body of knowledge.

What is the purpose of literature review?

A literature review serves several important purposes within academic and research contexts. Here are some key objectives and functions of a literature review: 2  

1. Contextualizing the Research Problem: The literature review provides a background and context for the research problem under investigation. It helps to situate the study within the existing body of knowledge. 

2. Identifying Gaps in Knowledge: By identifying gaps, contradictions, or areas requiring further research, the researcher can shape the research question and justify the significance of the study. This is crucial for ensuring that the new research contributes something novel to the field. 

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3. Understanding Theoretical and Conceptual Frameworks: Literature reviews help researchers gain an understanding of the theoretical and conceptual frameworks used in previous studies. This aids in the development of a theoretical framework for the current research. 

4. Providing Methodological Insights: Another purpose of literature reviews is that it allows researchers to learn about the methodologies employed in previous studies. This can help in choosing appropriate research methods for the current study and avoiding pitfalls that others may have encountered. 

5. Establishing Credibility: A well-conducted literature review demonstrates the researcher’s familiarity with existing scholarship, establishing their credibility and expertise in the field. It also helps in building a solid foundation for the new research. 

6. Informing Hypotheses or Research Questions: The literature review guides the formulation of hypotheses or research questions by highlighting relevant findings and areas of uncertainty in existing literature. 

Literature review example

Let’s delve deeper with a literature review example: Let’s say your literature review is about the impact of climate change on biodiversity. You might format your literature review into sections such as the effects of climate change on habitat loss and species extinction, phenological changes, and marine biodiversity. Each section would then summarize and analyze relevant studies in those areas, highlighting key findings and identifying gaps in the research. The review would conclude by emphasizing the need for further research on specific aspects of the relationship between climate change and biodiversity. The following literature review template provides a glimpse into the recommended literature review structure and content, demonstrating how research findings are organized around specific themes within a broader topic. 

Literature Review on Climate Change Impacts on Biodiversity:

Climate change is a global phenomenon with far-reaching consequences, including significant impacts on biodiversity. This literature review synthesizes key findings from various studies: 

a. Habitat Loss and Species Extinction:

Climate change-induced alterations in temperature and precipitation patterns contribute to habitat loss, affecting numerous species (Thomas et al., 2004). The review discusses how these changes increase the risk of extinction, particularly for species with specific habitat requirements. 

b. Range Shifts and Phenological Changes:

Observations of range shifts and changes in the timing of biological events (phenology) are documented in response to changing climatic conditions (Parmesan & Yohe, 2003). These shifts affect ecosystems and may lead to mismatches between species and their resources. 

c. Ocean Acidification and Coral Reefs:

The review explores the impact of climate change on marine biodiversity, emphasizing ocean acidification’s threat to coral reefs (Hoegh-Guldberg et al., 2007). Changes in pH levels negatively affect coral calcification, disrupting the delicate balance of marine ecosystems. 

d. Adaptive Strategies and Conservation Efforts:

Recognizing the urgency of the situation, the literature review discusses various adaptive strategies adopted by species and conservation efforts aimed at mitigating the impacts of climate change on biodiversity (Hannah et al., 2007). It emphasizes the importance of interdisciplinary approaches for effective conservation planning. 

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Writing a literature review involves summarizing and synthesizing existing research on a particular topic. A good literature review format should include the following elements. 

Introduction: The introduction sets the stage for your literature review, providing context and introducing the main focus of your review. 

• Opening Statement: Begin with a general statement about the broader topic and its significance in the field. 
• Scope and Purpose: Clearly define the scope of your literature review. Explain the specific research question or objective you aim to address. 
• Organizational Framework: Briefly outline the structure of your literature review, indicating how you will categorize and discuss the existing research. 
• Significance of the Study: Highlight why your literature review is important and how it contributes to the understanding of the chosen topic. 
• Thesis Statement: Conclude the introduction with a concise thesis statement that outlines the main argument or perspective you will develop in the body of the literature review. 

Body: The body of the literature review is where you provide a comprehensive analysis of existing literature, grouping studies based on themes, methodologies, or other relevant criteria. 

• Organize by Theme or Concept: Group studies that share common themes, concepts, or methodologies. Discuss each theme or concept in detail, summarizing key findings and identifying gaps or areas of disagreement. 
• Critical Analysis: Evaluate the strengths and weaknesses of each study. Discuss the methodologies used, the quality of evidence, and the overall contribution of each work to the understanding of the topic. 
• Synthesis of Findings: Synthesize the information from different studies to highlight trends, patterns, or areas of consensus in the literature. 
• Identification of Gaps: Discuss any gaps or limitations in the existing research and explain how your review contributes to filling these gaps. 
• Transition between Sections: Provide smooth transitions between different themes or concepts to maintain the flow of your literature review. 

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Conclusion: The conclusion of your literature review should summarize the main findings, highlight the contributions of the review, and suggest avenues for future research. 

• Summary of Key Findings: Recap the main findings from the literature and restate how they contribute to your research question or objective. 
• Contributions to the Field: Discuss the overall contribution of your literature review to the existing knowledge in the field. 
• Implications and Applications: Explore the practical implications of the findings and suggest how they might impact future research or practice. 
• Recommendations for Future Research: Identify areas that require further investigation and propose potential directions for future research in the field. 
• Final Thoughts: Conclude with a final reflection on the importance of your literature review and its relevance to the broader academic community. 

Conducting a literature review

Conducting a literature review is an essential step in research that involves reviewing and analyzing existing literature on a specific topic. It’s important to know how to do a literature review effectively, so here are the steps to follow: 1  

Choose a Topic and Define the Research Question:

• Select a topic that is relevant to your field of study. 
• Clearly define your research question or objective. Determine what specific aspect of the topic do you want to explore? 

Decide on the Scope of Your Review:

• Determine the timeframe for your literature review. Are you focusing on recent developments, or do you want a historical overview? 
• Consider the geographical scope. Is your review global, or are you focusing on a specific region? 
• Define the inclusion and exclusion criteria. What types of sources will you include? Are there specific types of studies or publications you will exclude? 

Select Databases for Searches:

• Identify relevant databases for your field. Examples include PubMed, IEEE Xplore, Scopus, Web of Science, and Google Scholar. 
• Consider searching in library catalogs, institutional repositories, and specialized databases related to your topic. 

Conduct Searches and Keep Track:

• Develop a systematic search strategy using keywords, Boolean operators (AND, OR, NOT), and other search techniques. 
• Record and document your search strategy for transparency and replicability. 
• Keep track of the articles, including publication details, abstracts, and links. Use citation management tools like EndNote, Zotero, or Mendeley to organize your references. 

Review the Literature:

• Evaluate the relevance and quality of each source. Consider the methodology, sample size, and results of studies. 
• Organize the literature by themes or key concepts. Identify patterns, trends, and gaps in the existing research. 
• Summarize key findings and arguments from each source. Compare and contrast different perspectives. 
• Identify areas where there is a consensus in the literature and where there are conflicting opinions. 
• Provide critical analysis and synthesis of the literature. What are the strengths and weaknesses of existing research? 

Organize and Write Your Literature Review:

• Literature review outline should be based on themes, chronological order, or methodological approaches. 
• Write a clear and coherent narrative that synthesizes the information gathered. 
• Use proper citations for each source and ensure consistency in your citation style (APA, MLA, Chicago, etc.). 
• Conclude your literature review by summarizing key findings, identifying gaps, and suggesting areas for future research. 

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Frequently asked questions

A literature review is a critical and comprehensive analysis of existing literature (published and unpublished works) on a specific topic or research question and provides a synthesis of the current state of knowledge in a particular field. A well-conducted literature review is crucial for researchers to build upon existing knowledge, avoid duplication of efforts, and contribute to the advancement of their field. It also helps researchers situate their work within a broader context and facilitates the development of a sound theoretical and conceptual framework for their studies.

Literature review is a crucial component of research writing, providing a solid background for a research paper’s investigation. The aim is to keep professionals up to date by providing an understanding of ongoing developments within a specific field, including research methods, and experimental techniques used in that field, and present that knowledge in the form of a written report. Also, the depth and breadth of the literature review emphasizes the credibility of the scholar in his or her field.  

Before writing a literature review, it’s essential to undertake several preparatory steps to ensure that your review is well-researched, organized, and focused. This includes choosing a topic of general interest to you and doing exploratory research on that topic, writing an annotated bibliography, and noting major points, especially those that relate to the position you have taken on the topic. 

Literature reviews and academic research papers are essential components of scholarly work but serve different purposes within the academic realm. 3 A literature review aims to provide a foundation for understanding the current state of research on a particular topic, identify gaps or controversies, and lay the groundwork for future research. Therefore, it draws heavily from existing academic sources, including books, journal articles, and other scholarly publications. In contrast, an academic research paper aims to present new knowledge, contribute to the academic discourse, and advance the understanding of a specific research question. Therefore, it involves a mix of existing literature (in the introduction and literature review sections) and original data or findings obtained through research methods. 

Literature reviews are essential components of academic and research papers, and various strategies can be employed to conduct them effectively. If you want to know how to write a literature review for a research paper, here are four common approaches that are often used by researchers.  Chronological Review: This strategy involves organizing the literature based on the chronological order of publication. It helps to trace the development of a topic over time, showing how ideas, theories, and research have evolved.  Thematic Review: Thematic reviews focus on identifying and analyzing themes or topics that cut across different studies. Instead of organizing the literature chronologically, it is grouped by key themes or concepts, allowing for a comprehensive exploration of various aspects of the topic.  Methodological Review: This strategy involves organizing the literature based on the research methods employed in different studies. It helps to highlight the strengths and weaknesses of various methodologies and allows the reader to evaluate the reliability and validity of the research findings.  Theoretical Review: A theoretical review examines the literature based on the theoretical frameworks used in different studies. This approach helps to identify the key theories that have been applied to the topic and assess their contributions to the understanding of the subject.  It’s important to note that these strategies are not mutually exclusive, and a literature review may combine elements of more than one approach. The choice of strategy depends on the research question, the nature of the literature available, and the goals of the review. Additionally, other strategies, such as integrative reviews or systematic reviews, may be employed depending on the specific requirements of the research.

The literature review format can vary depending on the specific publication guidelines. However, there are some common elements and structures that are often followed. Here is a general guideline for the format of a literature review:  Introduction:   Provide an overview of the topic.  Define the scope and purpose of the literature review.  State the research question or objective.  Body:   Organize the literature by themes, concepts, or chronology.  Critically analyze and evaluate each source.  Discuss the strengths and weaknesses of the studies.  Highlight any methodological limitations or biases.  Identify patterns, connections, or contradictions in the existing research.  Conclusion:   Summarize the key points discussed in the literature review.  Highlight the research gap.  Address the research question or objective stated in the introduction.  Highlight the contributions of the review and suggest directions for future research.

Both annotated bibliographies and literature reviews involve the examination of scholarly sources. While annotated bibliographies focus on individual sources with brief annotations, literature reviews provide a more in-depth, integrated, and comprehensive analysis of existing literature on a specific topic. The key differences are as follows: 

References 

• Denney, A. S., & Tewksbury, R. (2013). How to write a literature review.  Journal of criminal justice education ,  24 (2), 218-234. 
• Pan, M. L. (2016).  Preparing literature reviews: Qualitative and quantitative approaches . Taylor & Francis. 
• Cantero, C. (2019). How to write a literature review.  San José State University Writing Center . 

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• USC Libraries
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Organizing Your Social Sciences Research Paper

• 5. The Literature Review
• Purpose of Guide
• Design Flaws to Avoid
• Independent and Dependent Variables
• Glossary of Research Terms
• Reading Research Effectively
• Narrowing a Topic Idea
• Broadening a Topic Idea
• Extending the Timeliness of a Topic Idea
• Academic Writing Style
• Applying Critical Thinking
• Choosing a Title
• Making an Outline
• Paragraph Development
• Research Process Video Series
• Executive Summary
• The C.A.R.S. Model
• Background Information
• The Research Problem/Question
• Theoretical Framework
• Citation Tracking
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• Evaluating Sources
• Primary Sources
• Secondary Sources
• Tiertiary Sources
• Scholarly vs. Popular Publications
• Qualitative Methods
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• Using Non-Textual Elements
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• Common Grammar Mistakes
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• Further Readings
• Generative AI and Writing
• USC Libraries Tutorials and Other Guides
• Bibliography

A literature review surveys prior research published in books, scholarly articles, and any other sources relevant to a particular issue, area of research, or theory, and by so doing, provides a description, summary, and critical evaluation of these works in relation to the research problem being investigated. Literature reviews are designed to provide an overview of sources you have used in researching a particular topic and to demonstrate to your readers how your research fits within existing scholarship about the topic.

Fink, Arlene. Conducting Research Literature Reviews: From the Internet to Paper . Fourth edition. Thousand Oaks, CA: SAGE, 2014.

Importance of a Good Literature Review

A literature review may consist of simply a summary of key sources, but in the social sciences, a literature review usually has an organizational pattern and combines both summary and synthesis, often within specific conceptual categories . A summary is a recap of the important information of the source, but a synthesis is a re-organization, or a reshuffling, of that information in a way that informs how you are planning to investigate a research problem. The analytical features of a literature review might:

• Give a new interpretation of old material or combine new with old interpretations,
• Trace the intellectual progression of the field, including major debates,
• Depending on the situation, evaluate the sources and advise the reader on the most pertinent or relevant research, or
• Usually in the conclusion of a literature review, identify where gaps exist in how a problem has been researched to date.

Given this, the purpose of a literature review is to:

• Place each work in the context of its contribution to understanding the research problem being studied.
• Describe the relationship of each work to the others under consideration.
• Identify new ways to interpret prior research.
• Reveal any gaps that exist in the literature.
• Resolve conflicts amongst seemingly contradictory previous studies.
• Identify areas of prior scholarship to prevent duplication of effort.
• Point the way in fulfilling a need for additional research.
• Locate your own research within the context of existing literature [very important].

Fink, Arlene. Conducting Research Literature Reviews: From the Internet to Paper. 2nd ed. Thousand Oaks, CA: Sage, 2005; Hart, Chris. Doing a Literature Review: Releasing the Social Science Research Imagination . Thousand Oaks, CA: Sage Publications, 1998; Jesson, Jill. Doing Your Literature Review: Traditional and Systematic Techniques . Los Angeles, CA: SAGE, 2011; Knopf, Jeffrey W. "Doing a Literature Review." PS: Political Science and Politics 39 (January 2006): 127-132; Ridley, Diana. The Literature Review: A Step-by-Step Guide for Students . 2nd ed. Los Angeles, CA: SAGE, 2012.

Types of Literature Reviews

It is important to think of knowledge in a given field as consisting of three layers. First, there are the primary studies that researchers conduct and publish. Second are the reviews of those studies that summarize and offer new interpretations built from and often extending beyond the primary studies. Third, there are the perceptions, conclusions, opinion, and interpretations that are shared informally among scholars that become part of the body of epistemological traditions within the field.

In composing a literature review, it is important to note that it is often this third layer of knowledge that is cited as "true" even though it often has only a loose relationship to the primary studies and secondary literature reviews. Given this, while literature reviews are designed to provide an overview and synthesis of pertinent sources you have explored, there are a number of approaches you could adopt depending upon the type of analysis underpinning your study.

Argumentative Review This form examines literature selectively in order to support or refute an argument, deeply embedded assumption, or philosophical problem already established in the literature. The purpose is to develop a body of literature that establishes a contrarian viewpoint. Given the value-laden nature of some social science research [e.g., educational reform; immigration control], argumentative approaches to analyzing the literature can be a legitimate and important form of discourse. However, note that they can also introduce problems of bias when they are used to make summary claims of the sort found in systematic reviews [see below].

Integrative Review Considered a form of research that reviews, critiques, and synthesizes representative literature on a topic in an integrated way such that new frameworks and perspectives on the topic are generated. The body of literature includes all studies that address related or identical hypotheses or research problems. A well-done integrative review meets the same standards as primary research in regard to clarity, rigor, and replication. This is the most common form of review in the social sciences.

Historical Review Few things rest in isolation from historical precedent. Historical literature reviews focus on examining research throughout a period of time, often starting with the first time an issue, concept, theory, phenomena emerged in the literature, then tracing its evolution within the scholarship of a discipline. The purpose is to place research in a historical context to show familiarity with state-of-the-art developments and to identify the likely directions for future research.

Methodological Review A review does not always focus on what someone said [findings], but how they came about saying what they say [method of analysis]. Reviewing methods of analysis provides a framework of understanding at different levels [i.e. those of theory, substantive fields, research approaches, and data collection and analysis techniques], how researchers draw upon a wide variety of knowledge ranging from the conceptual level to practical documents for use in fieldwork in the areas of ontological and epistemological consideration, quantitative and qualitative integration, sampling, interviewing, data collection, and data analysis. This approach helps highlight ethical issues which you should be aware of and consider as you go through your own study.

Systematic Review This form consists of an overview of existing evidence pertinent to a clearly formulated research question, which uses pre-specified and standardized methods to identify and critically appraise relevant research, and to collect, report, and analyze data from the studies that are included in the review. The goal is to deliberately document, critically evaluate, and summarize scientifically all of the research about a clearly defined research problem . Typically it focuses on a very specific empirical question, often posed in a cause-and-effect form, such as "To what extent does A contribute to B?" This type of literature review is primarily applied to examining prior research studies in clinical medicine and allied health fields, but it is increasingly being used in the social sciences.

Theoretical Review The purpose of this form is to examine the corpus of theory that has accumulated in regard to an issue, concept, theory, phenomena. The theoretical literature review helps to establish what theories already exist, the relationships between them, to what degree the existing theories have been investigated, and to develop new hypotheses to be tested. Often this form is used to help establish a lack of appropriate theories or reveal that current theories are inadequate for explaining new or emerging research problems. The unit of analysis can focus on a theoretical concept or a whole theory or framework.

NOTE : Most often the literature review will incorporate some combination of types. For example, a review that examines literature supporting or refuting an argument, assumption, or philosophical problem related to the research problem will also need to include writing supported by sources that establish the history of these arguments in the literature.

Baumeister, Roy F. and Mark R. Leary. "Writing Narrative Literature Reviews."  Review of General Psychology 1 (September 1997): 311-320; Mark R. Fink, Arlene. Conducting Research Literature Reviews: From the Internet to Paper . 2nd ed. Thousand Oaks, CA: Sage, 2005; Hart, Chris. Doing a Literature Review: Releasing the Social Science Research Imagination . Thousand Oaks, CA: Sage Publications, 1998; Kennedy, Mary M. "Defining a Literature." Educational Researcher 36 (April 2007): 139-147; Petticrew, Mark and Helen Roberts. Systematic Reviews in the Social Sciences: A Practical Guide . Malden, MA: Blackwell Publishers, 2006; Torracro, Richard. "Writing Integrative Literature Reviews: Guidelines and Examples." Human Resource Development Review 4 (September 2005): 356-367; Rocco, Tonette S. and Maria S. Plakhotnik. "Literature Reviews, Conceptual Frameworks, and Theoretical Frameworks: Terms, Functions, and Distinctions." Human Ressource Development Review 8 (March 2008): 120-130; Sutton, Anthea. Systematic Approaches to a Successful Literature Review . Los Angeles, CA: Sage Publications, 2016.

Structure and Writing Style

I.  Thinking About Your Literature Review

The structure of a literature review should include the following in support of understanding the research problem :

• An overview of the subject, issue, or theory under consideration, along with the objectives of the literature review,
• Division of works under review into themes or categories [e.g. works that support a particular position, those against, and those offering alternative approaches entirely],
• An explanation of how each work is similar to and how it varies from the others,
• Conclusions as to which pieces are best considered in their argument, are most convincing of their opinions, and make the greatest contribution to the understanding and development of their area of research.

The critical evaluation of each work should consider :

• Provenance -- what are the author's credentials? Are the author's arguments supported by evidence [e.g. primary historical material, case studies, narratives, statistics, recent scientific findings]?
• Methodology -- were the techniques used to identify, gather, and analyze the data appropriate to addressing the research problem? Was the sample size appropriate? Were the results effectively interpreted and reported?
• Objectivity -- is the author's perspective even-handed or prejudicial? Is contrary data considered or is certain pertinent information ignored to prove the author's point?
• Persuasiveness -- which of the author's theses are most convincing or least convincing?
• Validity -- are the author's arguments and conclusions convincing? Does the work ultimately contribute in any significant way to an understanding of the subject?

II.  Development of the Literature Review

Four Basic Stages of Writing 1.  Problem formulation -- which topic or field is being examined and what are its component issues? 2.  Literature search -- finding materials relevant to the subject being explored. 3.  Data evaluation -- determining which literature makes a significant contribution to the understanding of the topic. 4.  Analysis and interpretation -- discussing the findings and conclusions of pertinent literature.

Consider the following issues before writing the literature review: Clarify If your assignment is not specific about what form your literature review should take, seek clarification from your professor by asking these questions: 1.  Roughly how many sources would be appropriate to include? 2.  What types of sources should I review (books, journal articles, websites; scholarly versus popular sources)? 3.  Should I summarize, synthesize, or critique sources by discussing a common theme or issue? 4.  Should I evaluate the sources in any way beyond evaluating how they relate to understanding the research problem? 5.  Should I provide subheadings and other background information, such as definitions and/or a history? Find Models Use the exercise of reviewing the literature to examine how authors in your discipline or area of interest have composed their literature review sections. Read them to get a sense of the types of themes you might want to look for in your own research or to identify ways to organize your final review. The bibliography or reference section of sources you've already read, such as required readings in the course syllabus, are also excellent entry points into your own research. Narrow the Topic The narrower your topic, the easier it will be to limit the number of sources you need to read in order to obtain a good survey of relevant resources. Your professor will probably not expect you to read everything that's available about the topic, but you'll make the act of reviewing easier if you first limit scope of the research problem. A good strategy is to begin by searching the USC Libraries Catalog for recent books about the topic and review the table of contents for chapters that focuses on specific issues. You can also review the indexes of books to find references to specific issues that can serve as the focus of your research. For example, a book surveying the history of the Israeli-Palestinian conflict may include a chapter on the role Egypt has played in mediating the conflict, or look in the index for the pages where Egypt is mentioned in the text. Consider Whether Your Sources are Current Some disciplines require that you use information that is as current as possible. This is particularly true in disciplines in medicine and the sciences where research conducted becomes obsolete very quickly as new discoveries are made. However, when writing a review in the social sciences, a survey of the history of the literature may be required. In other words, a complete understanding the research problem requires you to deliberately examine how knowledge and perspectives have changed over time. Sort through other current bibliographies or literature reviews in the field to get a sense of what your discipline expects. You can also use this method to explore what is considered by scholars to be a "hot topic" and what is not.

III.  Ways to Organize Your Literature Review

Chronology of Events If your review follows the chronological method, you could write about the materials according to when they were published. This approach should only be followed if a clear path of research building on previous research can be identified and that these trends follow a clear chronological order of development. For example, a literature review that focuses on continuing research about the emergence of German economic power after the fall of the Soviet Union. By Publication Order your sources by publication chronology, then, only if the order demonstrates a more important trend. For instance, you could order a review of literature on environmental studies of brown fields if the progression revealed, for example, a change in the soil collection practices of the researchers who wrote and/or conducted the studies. Thematic [“conceptual categories”] A thematic literature review is the most common approach to summarizing prior research in the social and behavioral sciences. Thematic reviews are organized around a topic or issue, rather than the progression of time, although the progression of time may still be incorporated into a thematic review. For example, a review of the Internet’s impact on American presidential politics could focus on the development of online political satire. While the study focuses on one topic, the Internet’s impact on American presidential politics, it would still be organized chronologically reflecting technological developments in media. The difference in this example between a "chronological" and a "thematic" approach is what is emphasized the most: themes related to the role of the Internet in presidential politics. Note that more authentic thematic reviews tend to break away from chronological order. A review organized in this manner would shift between time periods within each section according to the point being made. Methodological A methodological approach focuses on the methods utilized by the researcher. For the Internet in American presidential politics project, one methodological approach would be to look at cultural differences between the portrayal of American presidents on American, British, and French websites. Or the review might focus on the fundraising impact of the Internet on a particular political party. A methodological scope will influence either the types of documents in the review or the way in which these documents are discussed.

Other Sections of Your Literature Review Once you've decided on the organizational method for your literature review, the sections you need to include in the paper should be easy to figure out because they arise from your organizational strategy. In other words, a chronological review would have subsections for each vital time period; a thematic review would have subtopics based upon factors that relate to the theme or issue. However, sometimes you may need to add additional sections that are necessary for your study, but do not fit in the organizational strategy of the body. What other sections you include in the body is up to you. However, only include what is necessary for the reader to locate your study within the larger scholarship about the research problem.

Here are examples of other sections, usually in the form of a single paragraph, you may need to include depending on the type of review you write:

• Current Situation : Information necessary to understand the current topic or focus of the literature review.
• Sources Used : Describes the methods and resources [e.g., databases] you used to identify the literature you reviewed.
• History : The chronological progression of the field, the research literature, or an idea that is necessary to understand the literature review, if the body of the literature review is not already a chronology.
• Selection Methods : Criteria you used to select (and perhaps exclude) sources in your literature review. For instance, you might explain that your review includes only peer-reviewed [i.e., scholarly] sources.
• Standards : Description of the way in which you present your information.
• Questions for Further Research : What questions about the field has the review sparked? How will you further your research as a result of the review?

IV.  Writing Your Literature Review

Once you've settled on how to organize your literature review, you're ready to write each section. When writing your review, keep in mind these issues.

Use Evidence A literature review section is, in this sense, just like any other academic research paper. Your interpretation of the available sources must be backed up with evidence [citations] that demonstrates that what you are saying is valid. Be Selective Select only the most important points in each source to highlight in the review. The type of information you choose to mention should relate directly to the research problem, whether it is thematic, methodological, or chronological. Related items that provide additional information, but that are not key to understanding the research problem, can be included in a list of further readings . Use Quotes Sparingly Some short quotes are appropriate if you want to emphasize a point, or if what an author stated cannot be easily paraphrased. Sometimes you may need to quote certain terminology that was coined by the author, is not common knowledge, or taken directly from the study. Do not use extensive quotes as a substitute for using your own words in reviewing the literature. Summarize and Synthesize Remember to summarize and synthesize your sources within each thematic paragraph as well as throughout the review. Recapitulate important features of a research study, but then synthesize it by rephrasing the study's significance and relating it to your own work and the work of others. Keep Your Own Voice While the literature review presents others' ideas, your voice [the writer's] should remain front and center. For example, weave references to other sources into what you are writing but maintain your own voice by starting and ending the paragraph with your own ideas and wording. Use Caution When Paraphrasing When paraphrasing a source that is not your own, be sure to represent the author's information or opinions accurately and in your own words. Even when paraphrasing an author’s work, you still must provide a citation to that work.

V.  Common Mistakes to Avoid

These are the most common mistakes made in reviewing social science research literature.

• Sources in your literature review do not clearly relate to the research problem;
• You do not take sufficient time to define and identify the most relevant sources to use in the literature review related to the research problem;
• Relies exclusively on secondary analytical sources rather than including relevant primary research studies or data;
• Uncritically accepts another researcher's findings and interpretations as valid, rather than examining critically all aspects of the research design and analysis;
• Does not describe the search procedures that were used in identifying the literature to review;
• Reports isolated statistical results rather than synthesizing them in chi-squared or meta-analytic methods; and,
• Only includes research that validates assumptions and does not consider contrary findings and alternative interpretations found in the literature.

Cook, Kathleen E. and Elise Murowchick. “Do Literature Review Skills Transfer from One Course to Another?” Psychology Learning and Teaching 13 (March 2014): 3-11; Fink, Arlene. Conducting Research Literature Reviews: From the Internet to Paper . 2nd ed. Thousand Oaks, CA: Sage, 2005; Hart, Chris. Doing a Literature Review: Releasing the Social Science Research Imagination . Thousand Oaks, CA: Sage Publications, 1998; Jesson, Jill. Doing Your Literature Review: Traditional and Systematic Techniques . London: SAGE, 2011; Literature Review Handout. Online Writing Center. Liberty University; Literature Reviews. The Writing Center. University of North Carolina; Onwuegbuzie, Anthony J. and Rebecca Frels. Seven Steps to a Comprehensive Literature Review: A Multimodal and Cultural Approach . Los Angeles, CA: SAGE, 2016; Ridley, Diana. The Literature Review: A Step-by-Step Guide for Students . 2nd ed. Los Angeles, CA: SAGE, 2012; Randolph, Justus J. “A Guide to Writing the Dissertation Literature Review." Practical Assessment, Research, and Evaluation. vol. 14, June 2009; Sutton, Anthea. Systematic Approaches to a Successful Literature Review . Los Angeles, CA: Sage Publications, 2016; Taylor, Dena. The Literature Review: A Few Tips On Conducting It. University College Writing Centre. University of Toronto; Writing a Literature Review. Academic Skills Centre. University of Canberra.

Writing Tip

Break Out of Your Disciplinary Box!

Thinking interdisciplinarily about a research problem can be a rewarding exercise in applying new ideas, theories, or concepts to an old problem. For example, what might cultural anthropologists say about the continuing conflict in the Middle East? In what ways might geographers view the need for better distribution of social service agencies in large cities than how social workers might study the issue? You don’t want to substitute a thorough review of core research literature in your discipline for studies conducted in other fields of study. However, particularly in the social sciences, thinking about research problems from multiple vectors is a key strategy for finding new solutions to a problem or gaining a new perspective. Consult with a librarian about identifying research databases in other disciplines; almost every field of study has at least one comprehensive database devoted to indexing its research literature.

Frodeman, Robert. The Oxford Handbook of Interdisciplinarity . New York: Oxford University Press, 2010.

Another Writing Tip

Don't Just Review for Content!

While conducting a review of the literature, maximize the time you devote to writing this part of your paper by thinking broadly about what you should be looking for and evaluating. Review not just what scholars are saying, but how are they saying it. Some questions to ask:

• How are they organizing their ideas?
• What methods have they used to study the problem?
• What theories have been used to explain, predict, or understand their research problem?
• What sources have they cited to support their conclusions?
• How have they used non-textual elements [e.g., charts, graphs, figures, etc.] to illustrate key points?

When you begin to write your literature review section, you'll be glad you dug deeper into how the research was designed and constructed because it establishes a means for developing more substantial analysis and interpretation of the research problem.

Hart, Chris. Doing a Literature Review: Releasing the Social Science Research Imagination . Thousand Oaks, CA: Sage Publications, 1 998.

Yet Another Writing Tip

When Do I Know I Can Stop Looking and Move On?

Here are several strategies you can utilize to assess whether you've thoroughly reviewed the literature:

• Look for repeating patterns in the research findings . If the same thing is being said, just by different people, then this likely demonstrates that the research problem has hit a conceptual dead end. At this point consider: Does your study extend current research?  Does it forge a new path? Or, does is merely add more of the same thing being said?
• Look at sources the authors cite to in their work . If you begin to see the same researchers cited again and again, then this is often an indication that no new ideas have been generated to address the research problem.
• Search Google Scholar to identify who has subsequently cited leading scholars already identified in your literature review [see next sub-tab]. This is called citation tracking and there are a number of sources that can help you identify who has cited whom, particularly scholars from outside of your discipline. Here again, if the same authors are being cited again and again, this may indicate no new literature has been written on the topic.

Onwuegbuzie, Anthony J. and Rebecca Frels. Seven Steps to a Comprehensive Literature Review: A Multimodal and Cultural Approach . Los Angeles, CA: Sage, 2016; Sutton, Anthea. Systematic Approaches to a Successful Literature Review . Los Angeles, CA: Sage Publications, 2016.

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Literature Reviews

• 1. Define your research question
• Getting started
• Types of reviews

Define topic

Brainstorming, limit scope, how ai can help, helpful resources.

• 2. Plan your search
• 3. Search the literature
• 4. Organize your results
• 5. Synthesize your findings
• 6. Write the review
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For example, the following research question is too broad.

Does social media affect mental health in college students?

A more focused research question could be:

Does social media play a role in the number of eating disorder diagnoses in college-aged women?

Need some guidance? Contact your subject librarian for a research consultation or reach out to [email protected] .

Start the brainstorming process:

• Begin by identifying a broad topic of research interest (the more you're personally interested in it, the better!). Write it down to clarify your focus.
• Narrow down your scope  by considering specific areas within this topic that you want to explore.
• Identify key themes and elements  related to these specific areas. These will be the building blocks of your investigation.
• Finally,  integrate these key areas and elements  into a well-crafted research question.

One way to brainstorm is to use a flowchart.

The scope of a topic  encompasses the boundaries and constraints you set for your research. The more precise your research question, the smoother the research process becomes.

Feeling stuck? Here are some ways you can narrow down your topic:

• geographic location
• time period
• discipline/field of study
• age group, etc.

You:  I need to conduct a literature review about social media, mental health, and college students. What are some example research questions within this topic?

ChatGPT: 

• How does the frequency and nature of social media use impact the mental health of college students?
• What role do specific social media platforms play in influencing the mental well-being of college students?
• How do online social interactions on platforms like Facebook, Instagram, or Twitter relate to the mental health outcomes of college students?
• Are there differences in the association between social media use and mental health outcomes based on demographic factors such as gender, age, or academic major among college students?
• What strategies do college students employ to cope with negative mental health effects associated with social media use?
• How does the perception of social support through online platforms affect the mental health of college students?
• To what extent does the comparison of oneself to others on social media contribute to mental health challenges among college students?
• What interventions or educational programs are effective in promoting positive mental health behaviors in the context of social media use for college students?
• How do social media literacy and awareness programs impact the mental health attitudes and behaviors of college students?
• What is the relationship between the use of social media for academic and social purposes and the mental health outcomes of college students?

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For more information on how to incorporate AI tools into your research, check out the section on AI Tools .

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Conducting a Literature Review

Benefits of conducting a literature review.

• Steps in Conducting a Literature Review
• Summary of the Process
• Additional Resources
• Literature Review Tutorial by American University Library
• The Literature Review: A Few Tips On Conducting It by University of Toronto
• Write a Literature Review by UC Santa Cruz University Library

While there might be many reasons for conducting a literature review, following are four key outcomes of doing the review.

Assessment of the current state of research on a topic . This is probably the most obvious value of the literature review. Once a researcher has determined an area to work with for a research project, a search of relevant information sources will help determine what is already known about the topic and how extensively the topic has already been researched.

Identification of the experts on a particular topic . One of the additional benefits derived from doing the literature review is that it will quickly reveal which researchers have written the most on a particular topic and are, therefore, probably the experts on the topic. Someone who has written twenty articles on a topic or on related topics is more than likely more knowledgeable than someone who has written a single article. This same writer will likely turn up as a reference in most of the other articles written on the same topic. From the number of articles written by the author and the number of times the writer has been cited by other authors, a researcher will be able to assume that the particular author is an expert in the area and, thus, a key resource for consultation in the current research to be undertaken.

Identification of key questions about a topic that need further research . In many cases a researcher may discover new angles that need further exploration by reviewing what has already been written on a topic. For example, research may suggest that listening to music while studying might lead to better retention of ideas, but the research might not have assessed whether a particular style of music is more beneficial than another. A researcher who is interested in pursuing this topic would then do well to follow up existing studies with a new study, based on previous research, that tries to identify which styles of music are most beneficial to retention.

Determination of methodologies used in past studies of the same or similar topics.  It is often useful to review the types of studies that previous researchers have launched as a means of determining what approaches might be of most benefit in further developing a topic. By the same token, a review of previously conducted studies might lend itself to researchers determining a new angle for approaching research.

Upon completion of the literature review, a researcher should have a solid foundation of knowledge in the area and a good feel for the direction any new research should take. Should any additional questions arise during the course of the research, the researcher will know which experts to consult in order to quickly clear up those questions.

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Literature Review: Purpose of a Literature Review

• Literature Review
• Purpose of a Literature Review
• Work in Progress
• Compiling & Writing
• Books, Articles, & Web Pages
• Types of Literature Reviews
• Departmental Differences
• Citation Styles & Plagiarism
• Know the Difference! Systematic Review vs. Literature Review

The purpose of a literature review is to:

• Provide a foundation of knowledge on a topic
• Identify areas of prior scholarship to prevent duplication and give credit to other researchers
• Identify inconstancies: gaps in research, conflicts in previous studies, open questions left from other research
• Identify the need for additional research (justifying your research)
• Identify the relationship of works in the context of their contribution to the topic and other works
• Place your own research within the context of existing literature, making a case for why further study is needed.

Videos & Tutorials

VIDEO: What is the role of a literature review in research? What's it mean to "review" the literature? Get the big picture of what to expect as part of the process. This video is published under a Creative Commons 3.0 BY-NC-SA US license. License, credits, and contact information can be found here: https://www.lib.ncsu.edu/tutorials/litreview/

Elements in a Literature Review

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What is a literature review?

A literature review is an integrated analysis -- not just a summary-- of scholarly writings and other relevant evidence related directly to your research question.  That is, it represents a synthesis of the evidence that provides background information on your topic and shows a association between the evidence and your research question.

A literature review may be a stand alone work or the introduction to a larger research paper, depending on the assignment.  Rely heavily on the guidelines your instructor has given you.

Why is it important?

A literature review is important because it:

• Explains the background of research on a topic.
• Demonstrates why a topic is significant to a subject area.
• Discovers relationships between research studies/ideas.
• Identifies major themes, concepts, and researchers on a topic.
• Identifies critical gaps and points of disagreement.
• Discusses further research questions that logically come out of the previous studies.

APA7 Style resources

APA Style Blog - for those harder to find answers

1. Choose a topic. Define your research question.

Your literature review should be guided by your central research question.  The literature represents background and research developments related to a specific research question, interpreted and analyzed by you in a synthesized way.

• Make sure your research question is not too broad or too narrow.  Is it manageable?
• Begin writing down terms that are related to your question. These will be useful for searches later.
• If you have the opportunity, discuss your topic with your professor and your class mates.

2. Decide on the scope of your review

How many studies do you need to look at? How comprehensive should it be? How many years should it cover? 

• This may depend on your assignment.  How many sources does the assignment require?

3. Select the databases you will use to conduct your searches.

Make a list of the databases you will search. 

Where to find databases:

• use the tabs on this guide
• Find other databases in the Nursing Information Resources web page
• More on the Medical Library web page
• ... and more on the Yale University Library web page

4. Conduct your searches to find the evidence. Keep track of your searches.

• Use the key words in your question, as well as synonyms for those words, as terms in your search. Use the database tutorials for help.
• Save the searches in the databases. This saves time when you want to redo, or modify, the searches. It is also helpful to use as a guide is the searches are not finding any useful results.
• Review the abstracts of research studies carefully. This will save you time.
• Use the bibliographies and references of research studies you find to locate others.
• Check with your professor, or a subject expert in the field, if you are missing any key works in the field.
• Ask your librarian for help at any time.
• Use a citation manager, such as EndNote as the repository for your citations. See the EndNote tutorials for help.

Review the literature

Some questions to help you analyze the research:

• What was the research question of the study you are reviewing? What were the authors trying to discover?
• Was the research funded by a source that could influence the findings?
• What were the research methodologies? Analyze its literature review, the samples and variables used, the results, and the conclusions.
• Does the research seem to be complete? Could it have been conducted more soundly? What further questions does it raise?
• If there are conflicting studies, why do you think that is?
• How are the authors viewed in the field? Has this study been cited? If so, how has it been analyzed?

Tips: 

• Review the abstracts carefully.  
• Keep careful notes so that you may track your thought processes during the research process.
• Create a matrix of the studies for easy analysis, and synthesis, across all of the studies.
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Why is it important to do a literature review in research?

The importance of scientific communication in the healthcare industry

The Importance and Role of Biostatistics in Clinical Research

 “A substantive, thorough, sophisticated literature review is a precondition for doing substantive, thorough, sophisticated research”. Boote and Baile 2005

Authors of manuscripts treat writing a literature review as a routine work or a mere formality. But a seasoned one knows the purpose and importance of a well-written literature review.  Since it is one of the basic needs for researches at any level, they have to be done vigilantly. Only then the reader will know that the basics of research have not been neglected.

The aim of any literature review is to summarize and synthesize the arguments and ideas of existing knowledge in a particular field without adding any new contributions.   Being built on existing knowledge they help the researcher to even turn the wheels of the topic of research.  It is possible only with profound knowledge of what is wrong in the existing findings in detail to overpower them.  For other researches, the literature review gives the direction to be headed for its success. 

The common perception of literature review and reality:

As per the common belief, literature reviews are only a summary of the sources related to the research. And many authors of scientific manuscripts believe that they are only surveys of what are the researches are done on the chosen topic.  But on the contrary, it uses published information from pertinent and relevant sources like

• Scholarly books
• Scientific papers
• Latest studies in the field
• Established school of thoughts
• Relevant articles from renowned scientific journals

and many more for a field of study or theory or a particular problem to do the following:

• Summarize into a brief account of all information
• Synthesize the information by restructuring and reorganizing
• Critical evaluation of a concept or a school of thought or ideas
• Familiarize the authors to the extent of knowledge in the particular field
• Encapsulate
• Compare & contrast

By doing the above on the relevant information, it provides the reader of the scientific manuscript with the following for a better understanding of it:

• It establishes the authors’  in-depth understanding and knowledge of their field subject
• It gives the background of the research
• Portrays the scientific manuscript plan of examining the research result
• Illuminates on how the knowledge has changed within the field
• Highlights what has already been done in a particular field
• Information of the generally accepted facts, emerging and current state of the topic of research
• Identifies the research gap that is still unexplored or under-researched fields
• Demonstrates how the research fits within a larger field of study
• Provides an overview of the sources explored during the research of a particular topic

Importance of literature review in research:

The importance of literature review in scientific manuscripts can be condensed into an analytical feature to enable the multifold reach of its significance.  It adds value to the legitimacy of the research in many ways:

• Provides the interpretation of existing literature in light of updated developments in the field to help in establishing the consistency in knowledge and relevancy of existing materials
• It helps in calculating the impact of the latest information in the field by mapping their progress of knowledge.
• It brings out the dialects of contradictions between various thoughts within the field to establish facts
• The research gaps scrutinized initially are further explored to establish the latest facts of theories to add value to the field
• Indicates the current research place in the schema of a particular field
• Provides information for relevancy and coherency to check the research
• Apart from elucidating the continuance of knowledge, it also points out areas that require further investigation and thus aid as a starting point of any future research
• Justifies the research and sets up the research question
• Sets up a theoretical framework comprising the concepts and theories of the research upon which its success can be judged
• Helps to adopt a more appropriate methodology for the research by examining the strengths and weaknesses of existing research in the same field
• Increases the significance of the results by comparing it with the existing literature
• Provides a point of reference by writing the findings in the scientific manuscript
• Helps to get the due credit from the audience for having done the fact-finding and fact-checking mission in the scientific manuscripts
• The more the reference of relevant sources of it could increase more of its trustworthiness with the readers
• Helps to prevent plagiarism by tailoring and uniquely tweaking the scientific manuscript not to repeat other’s original idea
• By preventing plagiarism , it saves the scientific manuscript from rejection and thus also saves a lot of time and money
• Helps to evaluate, condense and synthesize gist in the author’s own words to sharpen the research focus
• Helps to compare and contrast to  show the originality and uniqueness of the research than that of the existing other researches
• Rationalizes the need for conducting the particular research in a specified field
• Helps to collect data accurately for allowing any new methodology of research than the existing ones
• Enables the readers of the manuscript to answer the following questions of its readers for its better chances for publication
• What do the researchers know?
• What do they not know?
• Is the scientific manuscript reliable and trustworthy?
• What are the knowledge gaps of the researcher?

22. It helps the readers to identify the following for further reading of the scientific manuscript:

• What has been already established, discredited and accepted in the particular field of research
• Areas of controversy and conflicts among different schools of thought
• Unsolved problems and issues in the connected field of research
• The emerging trends and approaches
• How the research extends, builds upon and leaves behind from the previous research

A profound literature review with many relevant sources of reference will enhance the chances of the scientific manuscript publication in renowned and reputed scientific journals .

References:

http://www.math.montana.edu/jobo/phdprep/phd6.pdf

journal Publishing services  |  Scientific Editing Services  |  Medical Writing Services  |  scientific research writing service  |  Scientific communication services

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Approaching literature review for academic purposes: The Literature Review Checklist

Debora f.b. leite.

I Departamento de Ginecologia e Obstetricia, Faculdade de Ciencias Medicas, Universidade Estadual de Campinas, Campinas, SP, BR

II Universidade Federal de Pernambuco, Pernambuco, PE, BR

III Hospital das Clinicas, Universidade Federal de Pernambuco, Pernambuco, PE, BR

Maria Auxiliadora Soares Padilha

Jose g. cecatti.

A sophisticated literature review (LR) can result in a robust dissertation/thesis by scrutinizing the main problem examined by the academic study; anticipating research hypotheses, methods and results; and maintaining the interest of the audience in how the dissertation/thesis will provide solutions for the current gaps in a particular field. Unfortunately, little guidance is available on elaborating LRs, and writing an LR chapter is not a linear process. An LR translates students’ abilities in information literacy, the language domain, and critical writing. Students in postgraduate programs should be systematically trained in these skills. Therefore, this paper discusses the purposes of LRs in dissertations and theses. Second, the paper considers five steps for developing a review: defining the main topic, searching the literature, analyzing the results, writing the review and reflecting on the writing. Ultimately, this study proposes a twelve-item LR checklist. By clearly stating the desired achievements, this checklist allows Masters and Ph.D. students to continuously assess their own progress in elaborating an LR. Institutions aiming to strengthen students’ necessary skills in critical academic writing should also use this tool.

INTRODUCTION

Writing the literature review (LR) is often viewed as a difficult task that can be a point of writer’s block and procrastination ( 1 ) in postgraduate life. Disagreements on the definitions or classifications of LRs ( 2 ) may confuse students about their purpose and scope, as well as how to perform an LR. Interestingly, at many universities, the LR is still an important element in any academic work, despite the more recent trend of producing scientific articles rather than classical theses.

The LR is not an isolated section of the thesis/dissertation or a copy of the background section of a research proposal. It identifies the state-of-the-art knowledge in a particular field, clarifies information that is already known, elucidates implications of the problem being analyzed, links theory and practice ( 3 - 5 ), highlights gaps in the current literature, and places the dissertation/thesis within the research agenda of that field. Additionally, by writing the LR, postgraduate students will comprehend the structure of the subject and elaborate on their cognitive connections ( 3 ) while analyzing and synthesizing data with increasing maturity.

At the same time, the LR transforms the student and hints at the contents of other chapters for the reader. First, the LR explains the research question; second, it supports the hypothesis, objectives, and methods of the research project; and finally, it facilitates a description of the student’s interpretation of the results and his/her conclusions. For scholars, the LR is an introductory chapter ( 6 ). If it is well written, it demonstrates the student’s understanding of and maturity in a particular topic. A sound and sophisticated LR can indicate a robust dissertation/thesis.

A consensus on the best method to elaborate a dissertation/thesis has not been achieved. The LR can be a distinct chapter or included in different sections; it can be part of the introduction chapter, part of each research topic, or part of each published paper ( 7 ). However, scholars view the LR as an integral part of the main body of an academic work because it is intrinsically connected to other sections ( Figure 1 ) and is frequently present. The structure of the LR depends on the conventions of a particular discipline, the rules of the department, and the student’s and supervisor’s areas of expertise, needs and interests.

Interestingly, many postgraduate students choose to submit their LR to peer-reviewed journals. As LRs are critical evaluations of current knowledge, they are indeed publishable material, even in the form of narrative or systematic reviews. However, systematic reviews have specific patterns 1 ( 8 ) that may not entirely fit with the questions posed in the dissertation/thesis. Additionally, the scope of a systematic review may be too narrow, and the strict criteria for study inclusion may omit important information from the dissertation/thesis. Therefore, this essay discusses the definition of an LR is and methods to develop an LR in the context of an academic dissertation/thesis. Finally, we suggest a checklist to evaluate an LR.

WHAT IS A LITERATURE REVIEW IN A THESIS?

Conducting research and writing a dissertation/thesis translates rational thinking and enthusiasm ( 9 ). While a strong body of literature that instructs students on research methodology, data analysis and writing scientific papers exists, little guidance on performing LRs is available. The LR is a unique opportunity to assess and contrast various arguments and theories, not just summarize them. The research results should not be discussed within the LR, but the postgraduate student tends to write a comprehensive LR while reflecting on his or her own findings ( 10 ).

Many people believe that writing an LR is a lonely and linear process. Supervisors or the institutions assume that the Ph.D. student has mastered the relevant techniques and vocabulary associated with his/her subject and conducts a self-reflection about previously published findings. Indeed, while elaborating the LR, the student should aggregate diverse skills, which mainly rely on his/her own commitment to mastering them. Thus, less supervision should be required ( 11 ). However, the parameters described above might not currently be the case for many students ( 11 , 12 ), and the lack of formal and systematic training on writing LRs is an important concern ( 11 ).

An institutional environment devoted to active learning will provide students the opportunity to continuously reflect on LRs, which will form a dialogue between the postgraduate student and the current literature in a particular field ( 13 ). Postgraduate students will be interpreting studies by other researchers, and, according to Hart (1998) ( 3 ), the outcomes of the LR in a dissertation/thesis include the following:

• To identify what research has been performed and what topics require further investigation in a particular field of knowledge;
• To determine the context of the problem;
• To recognize the main methodologies and techniques that have been used in the past;
• To place the current research project within the historical, methodological and theoretical context of a particular field;
• To identify significant aspects of the topic;
• To elucidate the implications of the topic;
• To offer an alternative perspective;
• To discern how the studied subject is structured;
• To improve the student’s subject vocabulary in a particular field; and
• To characterize the links between theory and practice.

A sound LR translates the postgraduate student’s expertise in academic and scientific writing: it expresses his/her level of comfort with synthesizing ideas ( 11 ). The LR reveals how well the postgraduate student has proceeded in three domains: an effective literature search, the language domain, and critical writing.

Effective literature search

All students should be trained in gathering appropriate data for specific purposes, and information literacy skills are a cornerstone. These skills are defined as “an individual’s ability to know when they need information, to identify information that can help them address the issue or problem at hand, and to locate, evaluate, and use that information effectively” ( 14 ). Librarian support is of vital importance in coaching the appropriate use of Boolean logic (AND, OR, NOT) and other tools for highly efficient literature searches (e.g., quotation marks and truncation), as is the appropriate management of electronic databases.

Language domain

Academic writing must be concise and precise: unnecessary words distract the reader from the essential content ( 15 ). In this context, reading about issues distant from the research topic ( 16 ) may increase students’ general vocabulary and familiarity with grammar. Ultimately, reading diverse materials facilitates and encourages the writing process itself.

Critical writing

Critical judgment includes critical reading, thinking and writing. It supposes a student’s analytical reflection about what he/she has read. The student should delineate the basic elements of the topic, characterize the most relevant claims, identify relationships, and finally contrast those relationships ( 17 ). Each scientific document highlights the perspective of the author, and students will become more confident in judging the supporting evidence and underlying premises of a study and constructing their own counterargument as they read more articles. A paucity of integration or contradictory perspectives indicates lower levels of cognitive complexity ( 12 ).

Thus, while elaborating an LR, the postgraduate student should achieve the highest category of Bloom’s cognitive skills: evaluation ( 12 ). The writer should not only summarize data and understand each topic but also be able to make judgments based on objective criteria, compare resources and findings, identify discrepancies due to methodology, and construct his/her own argument ( 12 ). As a result, the student will be sufficiently confident to show his/her own voice .

Writing a consistent LR is an intense and complex activity that reveals the training and long-lasting academic skills of a writer. It is not a lonely or linear process. However, students are unlikely to be prepared to write an LR if they have not mastered the aforementioned domains ( 10 ). An institutional environment that supports student learning is crucial.

Different institutions employ distinct methods to promote students’ learning processes. First, many universities propose modules to develop behind the scenes activities that enhance self-reflection about general skills (e.g., the skills we have mastered and the skills we need to develop further), behaviors that should be incorporated (e.g., self-criticism about one’s own thoughts), and each student’s role in the advancement of his/her field. Lectures or workshops about LRs themselves are useful because they describe the purposes of the LR and how it fits into the whole picture of a student’s work. These activities may explain what type of discussion an LR must involve, the importance of defining the correct scope, the reasons to include a particular resource, and the main role of critical reading.

Some pedagogic services that promote a continuous improvement in study and academic skills are equally important. Examples include workshops about time management, the accomplishment of personal objectives, active learning, and foreign languages for nonnative speakers. Additionally, opportunities to converse with other students promotes an awareness of others’ experiences and difficulties. Ultimately, the supervisor’s role in providing feedback and setting deadlines is crucial in developing students’ abilities and in strengthening students’ writing quality ( 12 ).

HOW SHOULD A LITERATURE REVIEW BE DEVELOPED?

A consensus on the appropriate method for elaborating an LR is not available, but four main steps are generally accepted: defining the main topic, searching the literature, analyzing the results, and writing ( 6 ). We suggest a fifth step: reflecting on the information that has been written in previous publications ( Figure 2 ).

First step: Defining the main topic

Planning an LR is directly linked to the research main question of the thesis and occurs in parallel to students’ training in the three domains discussed above. The planning stage helps organize ideas, delimit the scope of the LR ( 11 ), and avoid the wasting of time in the process. Planning includes the following steps:

• Reflecting on the scope of the LR: postgraduate students will have assumptions about what material must be addressed and what information is not essential to an LR ( 13 , 18 ). Cooper’s Taxonomy of Literature Reviews 2 systematizes the writing process through six characteristics and nonmutually exclusive categories. The focus refers to the reviewer’s most important points of interest, while the goals concern what students want to achieve with the LR. The perspective assumes answers to the student’s own view of the LR and how he/she presents a particular issue. The coverage defines how comprehensive the student is in presenting the literature, and the organization determines the sequence of arguments. The audience is defined as the group for whom the LR is written.
• Designating sections and subsections: Headings and subheadings should be specific, explanatory and have a coherent sequence throughout the text ( 4 ). They simulate an inverted pyramid, with an increasing level of reflection and depth of argument.
• Identifying keywords: The relevant keywords for each LR section should be listed to guide the literature search. This list should mirror what Hart (1998) ( 3 ) advocates as subject vocabulary . The keywords will also be useful when the student is writing the LR since they guide the reader through the text.
• Delineating the time interval and language of documents to be retrieved in the second step. The most recently published documents should be considered, but relevant texts published before a predefined cutoff year can be included if they are classic documents in that field. Extra care should be employed when translating documents.

Second step: Searching the literature

The ability to gather adequate information from the literature must be addressed in postgraduate programs. Librarian support is important, particularly for accessing difficult texts. This step comprises the following components:

• Searching the literature itself: This process consists of defining which databases (electronic or dissertation/thesis repositories), official documents, and books will be searched and then actively conducting the search. Information literacy skills have a central role in this stage. While searching electronic databases, controlled vocabulary (e.g., Medical Subject Headings, or MeSH, for the PubMed database) or specific standardized syntax rules may need to be applied.

In addition, two other approaches are suggested. First, a review of the reference list of each document might be useful for identifying relevant publications to be included and important opinions to be assessed. This step is also relevant for referencing the original studies and leading authors in that field. Moreover, students can directly contact the experts on a particular topic to consult with them regarding their experience or use them as a source of additional unpublished documents.

Before submitting a dissertation/thesis, the electronic search strategy should be repeated. This process will ensure that the most recently published papers will be considered in the LR.

• Selecting documents for inclusion: Generally, the most recent literature will be included in the form of published peer-reviewed papers. Assess books and unpublished material, such as conference abstracts, academic texts and government reports, are also important to assess since the gray literature also offers valuable information. However, since these materials are not peer-reviewed, we recommend that they are carefully added to the LR.

This task is an important exercise in time management. First, students should read the title and abstract to understand whether that document suits their purposes, addresses the research question, and helps develop the topic of interest. Then, they should scan the full text, determine how it is structured, group it with similar documents, and verify whether other arguments might be considered ( 5 ).

Third step: Analyzing the results

Critical reading and thinking skills are important in this step. This step consists of the following components:

• Reading documents: The student may read various texts in depth according to LR sections and subsections ( defining the main topic ), which is not a passive activity ( 1 ). Some questions should be asked to practice critical analysis skills, as listed below. Is the research question evident and articulated with previous knowledge? What are the authors’ research goals and theoretical orientations, and how do they interact? Are the authors’ claims related to other scholars’ research? Do the authors consider different perspectives? Was the research project designed and conducted properly? Are the results and discussion plausible, and are they consistent with the research objectives and methodology? What are the strengths and limitations of this work? How do the authors support their findings? How does this work contribute to the current research topic? ( 1 , 19 )
• Taking notes: Students who systematically take notes on each document are more readily able to establish similarities or differences with other documents and to highlight personal observations. This approach reinforces the student’s ideas about the next step and helps develop his/her own academic voice ( 1 , 13 ). Voice recognition software ( 16 ), mind maps ( 5 ), flowcharts, tables, spreadsheets, personal comments on the referenced texts, and note-taking apps are all available tools for managing these observations, and the student him/herself should use the tool that best improves his/her learning. Additionally, when a student is considering submitting an LR to a peer-reviewed journal, notes should be taken on the activities performed in all five steps to ensure that they are able to be replicated.

Fourth step: Writing

The recognition of when a student is able and ready to write after a sufficient period of reading and thinking is likely a difficult task. Some students can produce a review in a single long work session. However, as discussed above, writing is not a linear process, and students do not need to write LRs according to a specific sequence of sections. Writing an LR is a time-consuming task, and some scholars believe that a period of at least six months is sufficient ( 6 ). An LR, and academic writing in general, expresses the writer’s proper thoughts, conclusions about others’ work ( 6 , 10 , 13 , 16 ), and decisions about methods to progress in the chosen field of knowledge. Thus, each student is expected to present a different learning and writing trajectory.

In this step, writing methods should be considered; then, editing, citing and correct referencing should complete this stage, at least temporarily. Freewriting techniques may be a good starting point for brainstorming ideas and improving the understanding of the information that has been read ( 1 ). Students should consider the following parameters when creating an agenda for writing the LR: two-hour writing blocks (at minimum), with prespecified tasks that are possible to complete in one section; short (minutes) and long breaks (days or weeks) to allow sufficient time for mental rest and reflection; and short- and long-term goals to motivate the writing itself ( 20 ). With increasing experience, this scheme can vary widely, and it is not a straightforward rule. Importantly, each discipline has a different way of writing ( 1 ), and each department has its own preferred styles for citations and references.

Fifth step: Reflecting on the writing

In this step, the postgraduate student should ask him/herself the same questions as in the analyzing the results step, which can take more time than anticipated. Ambiguities, repeated ideas, and a lack of coherence may not be noted when the student is immersed in the writing task for long periods. The whole effort will likely be a work in progress, and continuous refinements in the written material will occur once the writing process has begun.

LITERATURE REVIEW CHECKLIST

In contrast to review papers, the LR of a dissertation/thesis should not be a standalone piece or work. Instead, it should present the student as a scholar and should maintain the interest of the audience in how that dissertation/thesis will provide solutions for the current gaps in a particular field.

A checklist for evaluating an LR is convenient for students’ continuous academic development and research transparency: it clearly states the desired achievements for the LR of a dissertation/thesis. Here, we present an LR checklist developed from an LR scoring rubric ( 11 ). For a critical analysis of an LR, we maintain the five categories but offer twelve criteria that are not scaled ( Figure 3 ). The criteria all have the same importance and are not mutually exclusive.

First category: Coverage

1. justified criteria exist for the inclusion and exclusion of literature in the review.

This criterion builds on the main topic and areas covered by the LR ( 18 ). While experts may be confident in retrieving and selecting literature, postgraduate students must convince their audience about the adequacy of their search strategy and their reasons for intentionally selecting what material to cover ( 11 ). References from different fields of knowledge provide distinct perspective, but narrowing the scope of coverage may be important in areas with a large body of existing knowledge.

Second category: Synthesis

2. a critical examination of the state of the field exists.

A critical examination is an assessment of distinct aspects in the field ( 1 ) along with a constructive argument. It is not a negative critique but an expression of the student’s understanding of how other scholars have added to the topic ( 1 ), and the student should analyze and contextualize contradictory statements. A writer’s personal bias (beliefs or political involvement) have been shown to influence the structure and writing of a document; therefore, the cultural and paradigmatic background guide how the theories are revised and presented ( 13 ). However, an honest judgment is important when considering different perspectives.

3. The topic or problem is clearly placed in the context of the broader scholarly literature

The broader scholarly literature should be related to the chosen main topic for the LR ( how to develop the literature review section). The LR can cover the literature from one or more disciplines, depending on its scope, but it should always offer a new perspective. In addition, students should be careful in citing and referencing previous publications. As a rule, original studies and primary references should generally be included. Systematic and narrative reviews present summarized data, and it may be important to cite them, particularly for issues that should be understood but do not require a detailed description. Similarly, quotations highlight the exact statement from another publication. However, excessive referencing may disclose lower levels of analysis and synthesis by the student.

4. The LR is critically placed in the historical context of the field

Situating the LR in its historical context shows the level of comfort of the student in addressing a particular topic. Instead of only presenting statements and theories in a temporal approach, which occasionally follows a linear timeline, the LR should authentically characterize the student’s academic work in the state-of-art techniques in their particular field of knowledge. Thus, the LR should reinforce why the dissertation/thesis represents original work in the chosen research field.

5. Ambiguities in definitions are considered and resolved

Distinct theories on the same topic may exist in different disciplines, and one discipline may consider multiple concepts to explain one topic. These misunderstandings should be addressed and contemplated. The LR should not synthesize all theories or concepts at the same time. Although this approach might demonstrate in-depth reading on a particular topic, it can reveal a student’s inability to comprehend and synthesize his/her research problem.

6. Important variables and phenomena relevant to the topic are articulated

The LR is a unique opportunity to articulate ideas and arguments and to purpose new relationships between them ( 10 , 11 ). More importantly, a sound LR will outline to the audience how these important variables and phenomena will be addressed in the current academic work. Indeed, the LR should build a bidirectional link with the remaining sections and ground the connections between all of the sections ( Figure 1 ).

7. A synthesized new perspective on the literature has been established

The LR is a ‘creative inquiry’ ( 13 ) in which the student elaborates his/her own discourse, builds on previous knowledge in the field, and describes his/her own perspective while interpreting others’ work ( 13 , 17 ). Thus, students should articulate the current knowledge, not accept the results at face value ( 11 , 13 , 17 ), and improve their own cognitive abilities ( 12 ).

Third category: Methodology

8. the main methodologies and research techniques that have been used in the field are identified and their advantages and disadvantages are discussed.

The LR is expected to distinguish the research that has been completed from investigations that remain to be performed, address the benefits and limitations of the main methods applied to date, and consider the strategies for addressing the expected limitations described above. While placing his/her research within the methodological context of a particular topic, the LR will justify the methodology of the study and substantiate the student’s interpretations.

9. Ideas and theories in the field are related to research methodologies

The audience expects the writer to analyze and synthesize methodological approaches in the field. The findings should be explained according to the strengths and limitations of previous research methods, and students must avoid interpretations that are not supported by the analyzed literature. This criterion translates to the student’s comprehension of the applicability and types of answers provided by different research methodologies, even those using a quantitative or qualitative research approach.

Fourth category: Significance

10. the scholarly significance of the research problem is rationalized.

The LR is an introductory section of a dissertation/thesis and will present the postgraduate student as a scholar in a particular field ( 11 ). Therefore, the LR should discuss how the research problem is currently addressed in the discipline being investigated or in different disciplines, depending on the scope of the LR. The LR explains the academic paradigms in the topic of interest ( 13 ) and methods to advance the field from these starting points. However, an excess number of personal citations—whether referencing the student’s research or studies by his/her research team—may reflect a narrow literature search and a lack of comprehensive synthesis of ideas and arguments.

11. The practical significance of the research problem is rationalized

The practical significance indicates a student’s comprehensive understanding of research terminology (e.g., risk versus associated factor), methodology (e.g., efficacy versus effectiveness) and plausible interpretations in the context of the field. Notably, the academic argument about a topic may not always reflect the debate in real life terms. For example, using a quantitative approach in epidemiology, statistically significant differences between groups do not explain all of the factors involved in a particular problem ( 21 ). Therefore, excessive faith in p -values may reflect lower levels of critical evaluation of the context and implications of a research problem by the student.

Fifth category: Rhetoric

12. the lr was written with a coherent, clear structure that supported the review.

This category strictly relates to the language domain: the text should be coherent and presented in a logical sequence, regardless of which organizational ( 18 ) approach is chosen. The beginning of each section/subsection should state what themes will be addressed, paragraphs should be carefully linked to each other ( 10 ), and the first sentence of each paragraph should generally summarize the content. Additionally, the student’s statements are clear, sound, and linked to other scholars’ works, and precise and concise language that follows standardized writing conventions (e.g., in terms of active/passive voice and verb tenses) is used. Attention to grammar, such as orthography and punctuation, indicates prudence and supports a robust dissertation/thesis. Ultimately, all of these strategies provide fluency and consistency for the text.

Although the scoring rubric was initially proposed for postgraduate programs in education research, we are convinced that this checklist is a valuable tool for all academic areas. It enables the monitoring of students’ learning curves and a concentrated effort on any criteria that are not yet achieved. For institutions, the checklist is a guide to support supervisors’ feedback, improve students’ writing skills, and highlight the learning goals of each program. These criteria do not form a linear sequence, but ideally, all twelve achievements should be perceived in the LR.

CONCLUSIONS

A single correct method to classify, evaluate and guide the elaboration of an LR has not been established. In this essay, we have suggested directions for planning, structuring and critically evaluating an LR. The planning of the scope of an LR and approaches to complete it is a valuable effort, and the five steps represent a rational starting point. An institutional environment devoted to active learning will support students in continuously reflecting on LRs, which will form a dialogue between the writer and the current literature in a particular field ( 13 ).

The completion of an LR is a challenging and necessary process for understanding one’s own field of expertise. Knowledge is always transitory, but our responsibility as scholars is to provide a critical contribution to our field, allowing others to think through our work. Good researchers are grounded in sophisticated LRs, which reveal a writer’s training and long-lasting academic skills. We recommend using the LR checklist as a tool for strengthening the skills necessary for critical academic writing.

AUTHOR CONTRIBUTIONS

Leite DFB has initially conceived the idea and has written the first draft of this review. Padilha MAS and Cecatti JG have supervised data interpretation and critically reviewed the manuscript. All authors have read the draft and agreed with this submission. Authors are responsible for all aspects of this academic piece.

ACKNOWLEDGMENTS

We are grateful to all of the professors of the ‘Getting Started with Graduate Research and Generic Skills’ module at University College Cork, Cork, Ireland, for suggesting and supporting this article. Funding: DFBL has granted scholarship from Brazilian Federal Agency for Support and Evaluation of Graduate Education (CAPES) to take part of her Ph.D. studies in Ireland (process number 88881.134512/2016-01). There is no participation from sponsors on authors’ decision to write or to submit this manuscript.

No potential conflict of interest was reported.

1 The questions posed in systematic reviews usually follow the ‘PICOS’ acronym: Population, Intervention, Comparison, Outcomes, Study design.

2 In 1988, Cooper proposed a taxonomy that aims to facilitate students’ and institutions’ understanding of literature reviews. Six characteristics with specific categories are briefly described: Focus: research outcomes, research methodologies, theories, or practices and applications; Goals: integration (generalization, conflict resolution, and linguistic bridge-building), criticism, or identification of central issues; Perspective: neutral representation or espousal of a position; Coverage: exhaustive, exhaustive with selective citations, representative, central or pivotal; Organization: historical, conceptual, or methodological; and Audience: specialized scholars, general scholars, practitioners or policymakers, or the general public.

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• Volume 19, Issue 1
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• Joanna Smith 1 ,
• Helen Noble 2
• 1 School of Healthcare, University of Leeds , Leeds , UK
• 2 School of Nursing and Midwifery, Queens's University Belfast , Belfast , UK
• Correspondence to Dr Joanna Smith , School of Healthcare, University of Leeds, Leeds LS2 9JT, UK; j.e.smith1{at}leeds.ac.uk

https://doi.org/10.1136/eb-2015-102252

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Implementing evidence into practice requires nurses to identify, critically appraise and synthesise research. This may require a comprehensive literature review: this article aims to outline the approaches and stages required and provides a working example of a published review.

Are there different approaches to undertaking a literature review?

What stages are required to undertake a literature review.

The rationale for the review should be established; consider why the review is important and relevant to patient care/safety or service delivery. For example, Noble et al 's 4 review sought to understand and make recommendations for practice and research in relation to dialysis refusal and withdrawal in patients with end-stage renal disease, an area of care previously poorly described. If appropriate, highlight relevant policies and theoretical perspectives that might guide the review. Once the key issues related to the topic, including the challenges encountered in clinical practice, have been identified formulate a clear question, and/or develop an aim and specific objectives. The type of review undertaken is influenced by the purpose of the review and resources available. However, the stages or methods used to undertake a review are similar across approaches and include:

Formulating clear inclusion and exclusion criteria, for example, patient groups, ages, conditions/treatments, sources of evidence/research designs;

Justifying data bases and years searched, and whether strategies including hand searching of journals, conference proceedings and research not indexed in data bases (grey literature) will be undertaken;

Developing search terms, the PICU (P: patient, problem or population; I: intervention; C: comparison; O: outcome) framework is a useful guide when developing search terms;

Developing search skills (eg, understanding Boolean Operators, in particular the use of AND/OR) and knowledge of how data bases index topics (eg, MeSH headings). Working with a librarian experienced in undertaking health searches is invaluable when developing a search.

Once studies are selected, the quality of the research/evidence requires evaluation. Using a quality appraisal tool, such as the Critical Appraisal Skills Programme (CASP) tools, 5 results in a structured approach to assessing the rigour of studies being reviewed. 3 Approaches to data synthesis for quantitative studies may include a meta-analysis (statistical analysis of data from multiple studies of similar designs that have addressed the same question), or findings can be reported descriptively. 6 Methods applicable for synthesising qualitative studies include meta-ethnography (themes and concepts from different studies are explored and brought together using approaches similar to qualitative data analysis methods), narrative summary, thematic analysis and content analysis. 7 Table 1 outlines the stages undertaken for a published review that summarised research about parents’ experiences of living with a child with a long-term condition. 8

• View inline

An example of rapid evidence assessment review

In summary, the type of literature review depends on the review purpose. For the novice reviewer undertaking a review can be a daunting and complex process; by following the stages outlined and being systematic a robust review is achievable. The importance of literature reviews should not be underestimated—they help summarise and make sense of an increasingly vast body of research promoting best evidence-based practice.

• ↵ Centre for Reviews and Dissemination . Guidance for undertaking reviews in health care . 3rd edn . York : CRD, York University , 2009 .
• ↵ Canadian Best Practices Portal. http://cbpp-pcpe.phac-aspc.gc.ca/interventions/selected-systematic-review-sites / ( accessed 7.8.2015 ).
• Bridges J , et al
• ↵ Critical Appraisal Skills Programme (CASP). http://www.casp-uk.net / ( accessed 7.8.2015 ).
• Dixon-Woods M ,
• Shaw R , et al
• Agarwal S ,
• Jones D , et al
• Cheater F ,

Twitter Follow Joanna Smith at @josmith175

Competing interests None declared.

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• Published: 14 May 2024

Effect of cytoplasmic fragmentation on embryo development, quality, and pregnancy outcome: a systematic review of the literature

• Ariella Yazdani 1 , 3 ,
• Iman Halvaei 2 ,
• Catherine Boniface 1 &
• Navid Esfandiari   ORCID: orcid.org/0000-0003-0979-5236 1 , 4  

Reproductive Biology and Endocrinology volume  22 , Article number:  55 ( 2024 ) Cite this article

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The role of cytoplasmic fragmentation in human embryo development and reproductive potential is widely recognized, albeit without standard definition nor agreed upon implication. While fragmentation is best understood to be a natural process across species, the origin of fragmentation remains incompletely understood and likely multifactorial. Several factors including embryo culture condition, gamete quality, aneuploidy, and abnormal cytokinesis seem to have important role in the etiology of cytoplasmic fragmentation. Fragmentation reduces the volume of cytoplasm and depletes embryo of essential organelles and regulatory proteins, compromising the developmental potential of the embryo. While it has been shown that degree of fragmentation and embryo implantation potential are inversely proportional, the degree, pattern, and distribution of fragmentation as it relates to pregnancy outcome is debated in the literature. This review highlights some of the challenges in analysis of fragmentation, while revealing trends in our evolving knowledge of how fragmentation may relate to functional development of the human embryos, implantation, and pregnancy outcome.

Introduction

Human preimplantation embryo scoring systems have been widely used to predict blastocyst development and implantation rate after in-vitro fertilization (IVF). The grading of embryos on day-2 and -3 after fertilization is largely subjective and interpretation varies across IVF laboratories, as it is commonly based on morphological appearance. Characteristics in early embryo grading schema include the amount of cytoplasmic fragmentation (CF) during early cleavage, speed of cellular division, number, size, and symmetry of cells (blastomeres). As defined by the Istanbul consensus workshop on embryo assessment, a fragment is “an extracellular membrane-bound cytoplasmic structure that is < 45 µm diameter in a day-2 embryo and < 40 µm diameter in a day-3 embryo” [ 1 ]. There are several different systems to evaluate embryo morphology including Hill’s scoring system [ 2 ] Cummins' grading system [ 3 ] ASEBIR grading system [ 1 ], the UK/ACE grading scheme [ 4 ]; each system has its own classification for degree of fragmentation as well as embryo grade. This heterogeneity further complicates analysis of fragmentation in relation to outcomes.

CF has been shown to occur early in embryonic division and is a common phenomenon seen in embryos cultured in vitro. CF has traditionally been used as a metric of embryo implantation potential [ 3 , 5 , 6 , 7 ]. The amount and pattern of fragments are analyzed in early development, incorporated into the embryo grade depending on grading system, and used to help select the most developmentally competent embryo to be transferred during an IVF cycle. This classification system is important as a proportion of embryos within a single cohort will not successfully develop to the blastocyst stage in vitro. Although there are various contributing factors to an embryo’s developmental capacity and viability, it is largely agreed upon that fragmentation plays an important role. It seems that the etiology of embryo fragmentation is not fully understood but it may be related to several factors like gamete quality, culture condition, and genetic abnormalities in the embryo [ 8 ]. It is difficult to directly compare and quantify relative degrees of fragmentation across studies. However, it has been repeatedly shown that the extent of fragmentation and implantation potential are inversely proportional [ 5 , 7 , 9 , 10 , 11 , 12 , 13 , 14 , 15 , 16 , 17 , 18 , 19 , 20 , 21 ]. While a low degree of fragmentation does not seem to significantly impact embryo viability, severe fragmentation does [ 7 , 22 , 23 ]. Alongside the cell to cytoplasmic ratio, the pattern and distribution of fragmentation influence the developmental quality of the embryo [ 7 , 24 ]. There are two main patterns of embryo cytoplasmic fragments: scattered and concentrated. The former is characterized by fragment contact within several blastomeres and is related to aneuploidy [ 25 ]. Time-lapse studies have shown that fragmentation is thought to be a dynamic process, where some fragments can be expelled or reintroduced into the cells as the embryo continues to divide [ 25 , 26 ]. Fragments can also easily move or rotate around the associated blastomere and change their position in the embryo [ 27 ].

Current grading systems used to evaluate cleavage-stage embryos are largely based on day-2 or -3 morphology. This can be problematic, as developmental growth of an embryo is variable and the grade of a developing embryo at one point in time is not guaranteed to persist. For example, studies have suggested that embryo selection on day-2 or -3 based on morphological grade can be unreliable and lead to negative pregnancy outcomes [ 28 , 29 , 30 ]. Accordingly, new parameters for predicting implantation success have been proposed including extended embryo culture to the blastocyst stage to day-5, -6 or -7 [ 31 ]. Delaying embryo transfer to the blastocyst stage is advantageous as it can limit the number of unsuccessful embryo transfers and biochemical pregnancies or clinical pregnancy losses in IVF. While there are multiple reports on the impact of cleavage-stage embryo quality on blastocyst formation and blastocyst quality [ 32 , 33 ], few have specifically looked at the degree of fragmentation as a predictive variable.

In this systematic review, we comprehensively reviewed the available literature on the origin and characteristics of CF, factors affecting CF, and the effect of CF and fragment removal on embryo development and pregnancy rate.

Materials and methods

A search was conducted on October 10, 2023, using PubMed and Google Scholar databases in accordance with Preferred Reporting Items for Systematic Reviews and Meta-Analysis guidelines [ 34 ]. In PubMed, the search terms “embryo*[tw] OR cleavage stage [tw] OR "Embryonic Structures"[Mesh] OR "Embryonic Development"[Mesh] OR "Embryo, Mammalian"[Mesh] OR "Cleavage Stage, Ovum"[Mesh]” AND “cytoplasm*[tw] AND fragment*[tw] AND “(Blastocyst*[tw] OR "Blastocyst"[Mesh]) AND (form* OR develop* OR quality*)” were used. A title search in Google Scholar using search terms as above and “embryo cytoplasm fragmentation”, “blastocyst quality”, “blastocyst development” was performed. Only full-text publications in English were included. Full-text articles which did not have any mention of cytoplasmic or embryo fragmentation were excluded, however articles which mentioned both DNA fragmentation and CF were included. Since most of the studies discussing CF also discussed other morphologic features of the embryo, studies that mention embryo morphology, grade or quality were also included. Articles that looked at non-human embryo fragmentation, case reports, case series, book chapters and review papers were excluded. Titles and abstracts were screened, and study quality and bias were assessed. The primary outcomes of interest were embryo quality, blastocyst formation, and pregnancy outcome.

Figure 1 provides details of study screening and inclusion. There were 206 studies screened between the two search engines PubMed ( n =106) and Google Scholar ( n =100). There were 18 duplicates giving a total of 188 articles. Due to the small number of studies from the search criteria, no filter of time was placed. After removal of non-full text articles, articles that used non-human embryos, and articles not relevant to the topic, 20 articles were eligible for inclusion. Forty relevant references from the articles were also extracted, reviewed, and included in this review. These additional articles were reviewed with the same inclusion and exclusion criteria as mentioned above. A total of 60 articles were included in the qualitative synthesis of this review.

Article Identification and Screening

Origin and etiology of CF

The etiology of CF is not completely understood. There are several proposed theories as to why embryos display variable degrees of fragmentation. Fragmentation has been shown to be a natural, unpredictable process both in vitro and in vivo and is documented in various species [ 35 , 36 ]. This suggests that embryo fragmentation is neither species-specific nor solely a byproduct of in vitro culture. Assisted reproductive technology (ART) and IVF techniques, such as time-lapse microscopy (TLM) and transmission electron microscopic (TEM) analyses, have recently allowed for further understanding of embryo developmental potential and fragmentation (Figs.  2 and 3 ). Seven of the included studies in this review propose potential hypotheses as to the origin of CF (Table 1 ). Three of the articles evaluated gamete quality as related to fragmentation in a developing embryo [ 37 , 38 , 39 ].

Human cleavage stage embryos a) Day-2 embryo at 4-cell stage with no fragmentation, b) fragmented Day-2 embryo, c) Day-3 embryo at 8-cell stage with no fragmentation, d) fragmented Day-3 embryo, e) Day-5 cavitating Morula with no fragmentation, f) fragmented Day-5 cavitating Morula

Ultrastructure and organelle microtopography of an embryo fragment by transmission electron microscopy. Ly: primary lysosome, M: mitochondrion, rM: remnant of regressing mitochondrion, MV: mitochondria-vesicle complex, V: vesicle; scale bar: 1 µM

An early study showed that sperm DNA oxidation has been associated with embryo development and quality, and therefore linked to CF [ 37 ]. Nucleolar asynchrony in the zygote from sperm DNA fragmentation has previously been shown to predict future low-quality blastocyst development. A positive correlation has also been found between the percentage of sperm OxiDNA-stained cells with embryo fragmentation on day-2 and -3 of development. Sperm DNA oxidation may therefore be associated with fragmented, nonviable, poor-quality embryos [ 37 ] . A recent study also showed the negative correlation between sperm DNA fragmentation and blastomere DNA fragmentation and blastulation rate [ 40 ]. Further studies are needed to confirm the impact of sperm DNA oxidation on embryo fragmentation.

An observational study documented the degree of fragmentation of human embryos as they progressed through mitotic cell cycles [ 38 ]. In this study, the authors analyzed nearly 2,000 oocytes and 372 embryos, and found that increased embryo fragmentation (>50%) was associated with a specific pattern of development: delayed first division (oocyte spindle detected at 36.2 hours after hCG injection vs. 35.5 hours in low fragmentation), a significantly earlier start of the second mitosis (8.9 hours vs. 10.8 hours after the first mitosis), and a significant delay of the third mitosis after the second mitosis (2.2. hours vs. 0.6 hours). The authors did not comment on whether fragmentation could be a result of the cell dividing before proper chromosome alignment, or if existing aneuploidy resulted in erroneous cleavage patterns [ 38 ].

Polar body (PB) fragmentation has also been investigated in relation to cytoplasmic fragmentation. Ebner et al., in a prospective study analyzed the relationship between a fragmented first PB and embryo quality in patients undergoing ICSI. Two groups of oocytes were analyzed according to PB fragmentation: intact first PBs and those with fragmented PBs. Forty-two hours after ICSI, embryo morphology (i.e., number of blastomeres and degree of fragmentation) was recorded. Overall, a significantly higher percentage of cytoplasmic fragmentation was seen in day-2 embryos that originated from oocytes with fragmented first PBs than those with intact PBs ( P < 0.05). This study further supports the concept that oocyte quality contributes to overall embryo fragmentation and provides evidence that preselection of oocytes may contribute to the prognosis of embryo quality and blastocyst development [ 39 ]. The role of PB fragmentation on embryo quality was confirmed in other studies [ 41 , 42 ], however, a recent study has not recommended considering PB status as a tool for embryo selection [ 43 ].

Beyond analysis of gamete quality, other studies have shown a biochemical relationship between embryo competence and fragmentation. One study showed that disturbances in E-cadherin, a cell adhesion protein that plays a critical role in morphogenesis, occur in embryos with cleavage abnormalities and extensive cytoplasmic fragmentation, suggesting a possible mechanism to the loss of embryonic viability [ 44 ]. Further, by using mitochondrial fluorescence techniques, Van Blerkom et al., found that mitochondrial distribution at the pronuclear stage may be an epigenetic factor related to the organization of the embryo and further embryonic development [ 45 ]. Blastomeres that were deficient in mitochondria and thus ATP at the first or second cell division remained undivided and often died during subsequent culture. Although this study examined morphologically normal (unfragmented) cleavage-stage embryos, it may support the idea that perinuclear mitochondrial distribution and microtubular organization influence developmental capacity of early cleavage-stage embryos [ 45 ]. Higher numbers of mitochondria reported in fragmented compared to the normal blastomeres show the rapid depletion of ATP in the fragmented embryos [ 21 ]. There have also been reports of increased gene transcription of mitochondrial factors like OXPHOS complexes, ATP synthase, and mtDNA content in highly fragmented embryos compared to controls [ 46 ]. Mitochondrial activity is lower and more centralized in fragmented embryos compared to good quality embryos on day-3 [ 47 ]. Mitochondria are the main source of ATP for embryo mitosis, and their proper function is essential for embryo development. More research is needed to elucidate the morphology and role of mitochondria in embryo development, especially in relation to fragmentation.

A subsequent study by Van Blerkom et al., analyzed the temporal and spatial aspects of fragmentation through TLM and TEM analyses from the pronuclear to the 10-12-cell stage. Through TLM, the authors visualized the non-discrete, dynamic nature of fragments and noted that many were “bleb-elaborations” of the plasma membrane and cytoplasm. They characterized two patterns of fragmentation: definitive and pseudo-fragmentation. Definitive fragmentation was described as fragments detached from a blastomere, and pseudo-fragmentation was assigned when the fragments were no longer detectable during subsequent development. Often one developing embryo would show both fragmentation patterns at different stages of development, suggesting that these patterns may have different etiologies and effects on embryo development competence [ 47 ]. Hardarson et al., similarly used TLM to document that fragments are dynamic and can be internalized throughout cleavage during culture periods. The contents of the fragments were noted to be internalized and released into the cytoplasm of the blastomere and seen on multiple time-lapse photographs as a cytoplasmic turbulence. This is the first reported evidence that cellular fragments can “disappear” during the culture period in human IVF [ 26 ]. It seems that in mild to moderate CF, the timing of embryo evaluation and grading can affect the reported percent of fragmentation.

Lastly, we have included a preliminary study performed by Sermondade et al., that suggests a specific subgroup of patients who have had repeated IVF failures (presumably due to a recurring high rate of fragmented embryos) may benefit from early intrauterine embryo transfer at the zygote stage (2PN) [ 48 ]. Data showed a delivery rate per oocyte retrieval of 18.9%, which was significantly higher than the delivery rate of 7.5% in the matched control group. The results were encouraging and suggestive of a safe, non-invasive rescue strategy for patients who experience recurrent highly fragmented embryos and failed IVF attempts. The data further suggests that fertilized oocytes of this subgroup may have deficiencies in certain maternal factors (i.e., stress-response factors) that do not allow normal embryo development in culture environments [ 48 ]. Another study was also confirmed application of zygote transfer in patients with history of low-quality embryos [ 49 ]. However, further studies are required to verify the impact of this technique for patients with history of fragmented embryos.

Apoptosis is another proposed etiology of fragmentation. Apoptosis may occur in blastomeres with defective cytoplasm or abnormal chromosomes, leading to embryo fragmentation [ 50 ]. There are several studies reporting apoptosis in both fragments and neighboring blastomeres in a fragmented embryo [ 24 , 50 ]. Chi et al., showed that fragments are associated with both apoptosis and necrosis [ 21 ]. One of the factors that appears to induce apoptosis in blastomeres is suboptimal culture conditions such as hypoxia [ 51 ]. In addition, there are controversial reports on the role of reactive oxygen species (ROS) in embryo fragmentation [ 52 , 53 ]. It has been shown that ROS are present at high levels in the culture media of fragmented embryos [ 52 , 54 ]. Chen et al., recently showed that embryo culture in 5% oxygen, from days 1 to 3, is associated with higher embryo quality and live birth rate compared to 20% oxygen [ 55 ]. The effects of culture condition modifications, such as hypoxia and ROS, on embryo fragmentation need to be clarified to understand the importance of culture condition in this process.

Membrane compartmentalization of DNA, abnormal cytokinesis, and extra vesicular formation are other proposed theories for embryo fragmentation [ 8 ]. Defects or damages in mitochondria are associated with low ATP and high ROS production leading to a compromised cell division and cytokinesis [ 27 ]. In addition, there is a correlation between embryo fragmentation and ploidy status. Chavez et al., showed that CF was seen in a high proportion of aneuploid embryos, and that meiotic and mitotic errors may cause fragmentation in different cell development stages. Meiotic errors were associated with fragmentation at one-cell stage while mitotic errors were associated with fragmentation at interphase or after first cytokinesis [ 56 ]. Chromosomally abnormal embryos often have severe fragmentation, which may be another cause of CF [ 55 , 57 ].

Overall, the precise cause of CF has yet to be clearly defined. The above investigations have elucidated potential sources and associations of what is likely a complex and multifactorial process and represent our current understanding of CF origin.

What is contained in CF?

Four of the included studies used various technological advances to study the contents of CF in human embryos (Table 2 ). Two studies used TEM methods to evaluate fragment ultrastructure (Fig.  3 ) [ 21 , 58 ]. Fragments were extracted from embryos with 10-50% fragmentation and the ultrastructure evaluated by TEM. Micrographs showed that the fragments had a distinct membrane containing cytoplasmic organelles including mitochondria, mitochondria-vesicle complexes, Golgi apparatus, primary lysosomes, and vacuoles. Mitochondria were the most abundant structure.

In an additional evaluation of CF contents, Johansson et al., analyzed DNA content of fragments to define a cutoff diameter for an anucleate fragment or blastomere. Findings showed that 98% of fragments <45 µm on day-2 and 97% of those <40 µm on day-3 contained no DNA and, if not reabsorbed into a blastomere, showed a loss of cytoplasm. Presence of essential blastomere organelles such as mitochondria, mRNA, and proteins within cytoplasmic fragments were related to embryo development arrest [ 59 ]. Lastly, Chi et al., also used TEM to examine ultrastructure of the human fragmented embryos and found that blastomeres with anucleate fragments contained fewer mitochondria in their cytoplasm compared to normal blastomeres [ 21 ].

Cell death and CF

Eight of the included studies analyzed the relationship between cell death and embryo fragmentation (Table 3 ). Five studies analyzed the status of chromatin in arrested fragmented embryos through a combined technique for simultaneous nuclear and terminal transferase-mediated DNA end labelling (TUNEL) [ 24 , 60 , 61 , 62 , 63 ]. Two studies used a comet assay to analyze DNA fragmentation [ 21 , 63 ]. Four of the eight studies used Annexin V staining [ 21 , 61 , 62 , 63 ] with three including the presence of propidium iodide (PI) to compare apoptosis to necrosis [ 21 , 61 , 63 ].

Jurisicova et al., used a combined nuclear and fragmented DNA labeling approach which allowed distinction between chromatin status and DNA fragmentation, which serve as markers of apoptosis versus necrosis respectively [ 60 ]. After fertilization, embryos were stained with 4,6-diamidino-2-phenylindole (DAPI). In cases of compromised cell membrane integrity, DAPI stain was observed in the cytoplasm as a sign of necrosis. Concomitant use of TUNEL labeling reflected the integrity of the DNA and allowed distinction between necrotic and apoptotic cells. Through combined techniques of DAPI/TUNEL, TEM, scanning electron microscopy (SEM) and stereomicroscopic observations, 153 of 203 (75.4%) fragmented early cleavage-stage embryos displayed signs of apoptosis (i.e., chromatin condensation, cellular shrinkage, DNA fragmentation, presence of cell corpses) with or without normal nuclei [ 60 ].

Similarly, Levy et al., analyzed early arrested or fragmented preimplantation embryos and the pattern of DNA fragmentation using TUNEL assay and the presence of phosphatidylserine through Fluorescein isothiocyanate (FITC)-labelled Annexin V, a phosphatidylserine binding protein. The authors observed TUNEL staining in one or more nuclei of 15 out of 50 (30%) arrested embryos from the 2-cell stage to uncompacted morulae, all of which had high degrees of CF. Furthermore, embryos with regular-sized blastomeres without fragmentation were all TUNEL negative [ 50 ].

A separate prospective study by Antczak et al., explored the possible association between fragmentation and apoptosis using PI and Annexin V staining of plasma membrane phosphatidylserine and TUNEL analysis of blastomere DNA [ 24 ]. In contradistinction to prior studies, these authors found no direct correlation between fragmentation and apoptosis. Virtually all blastomeres that were PI negative, intact or fragmented, showed no TUNEL or annexin V fluorescence, suggesting no signs of apoptosis [ 24 ].

Liu et al., used a similar methodology of TUNEL labeling and Annexin V staining to detect markers of apoptosis in fragmented human embryos derived from IVF [ 61 ]. Overall, highly fragmented embryos had apoptotic features including bright fluorescence (positive TUNEL labeling signifying DNA fragmentation) on the cell corpses and in intact blastomeres [ 61 ]. By staining cells with both annexin V and PI, this study was able to demonstrate that apoptosis occurs frequently in fragmented human embryos and the coexistence of apoptotic, necrotic and viable sibling blastomeres can occur. Sibling blastomeres within an embryo often showed apoptotic features that led to secondary necrosis while others did not initiate apoptosis. The authors did not find a significant difference in the expression frequency of apoptotic genes between viable and nonviable or arrested embryos [ 61 ].

Chi et al., stained human embryos ( n =10) with annexin V and PI and found that human fragmented embryos exhibited characteristics of both necrosis and apoptosis [ 20 ]. Rather than TUNEL assay, these authors used a modified sperm comet assay to investigate DNA fragmentation of human fragmented embryos. They found that 6/7 human fragmented embryos (85.1%) stained positively for PI with the intensity of staining increasing with the degree of fragmentation. Of note, DNA fragmentation was observed in fragmented human embryos but not in the normal embryo [ 21 ].

Metcalfe et al., analyzed the expression of 11 BCL-2 family genes in normally developing embryos and in severely fragmented embryos [ 64 ]. They found that the expression of BCL-2 family genes was highest in the pronuclear stage and eight-cell stages, and lowest at the two-cell, four-cell, and blastocyst stages in developmentally intact embryos. Furthermore, the expression did not change in fragmented embryos, suggesting that embryo fragmentation does not likely compromise mRNA integrity and gene detection [ 64 ]. However, like Liu et al., [ 61 ] these authors did detect far fewer pro-apoptotic BCL-2 genes in fragmented embryos at the eight-cell stage. The authors noted that these findings do not distinguish between iatrogenic apoptosis from suboptimal in-vitro culture conditions [ 64 ]. A separate study by Jurisicova et al. similarly analyzed gene expression at the 2-, 4- and 8-cell stage of fragmented embryos. Embryos that had 30-50% fragmentation showed a significant increase in Hrk mRNA levels, a BCL-2 protein encoding gene ( P = 0.016). Further, these authors found an increase in Caspase-3 mRNA in fragmented embryos, as well as induction of Caspase-3-like enzyme activity in nucleated fragments, although this finding was not statistically significant [ 65 ].

Van Blerkom et al., also used TUNEL assay in conjunction with the comet assay as a method of identifying the specific pattern of cell death (necrosis, lysis or apoptosis) and the extent of DNA damage in developing embryos [ 47 ]. They analyzed the integrity of the plasma membrane through annexin V staining with PI. They examined both transient and persistent fragment clusters at day-3 and 3.5 embryos for evidence of programed cell death using time-lapse video and TEM. In contrast to previous studies, they found no indication of nuclear DNA damage or loss of membrane integrity. These results, led the authors to hypothesize that the fragmentation observed was not characteristic of programed cell death, but rather resembled features of oncosis. The culture in this study was not severely oxygen-deprived and thus the authors concluded that this oncosis-like process was potentially a result of disproportionate mitochondrial segregation during the first cleavage division. Without sufficient mitochondria, the early blastomeres did not maintain adequate ATP for normal cell function which may have precipitated an ATP-driven oncosis-like process [ 47 ].

Lastly, a study by Bencomo et al., found correlations between the degree of apoptosis in human granulosa-lutein (GL) cells, the outcome of IVF-ET cycle, the percentage of embryo fragmentation, and patient’s age [ 66 ]. Human GL cells were collected from follicular fluid, cultured for 48 hours, and marked with caspACE FITC-VAD-FMK, a fluorescent marker for activated caspases. Results showed that GL cells of older women (>38 years old) were significantly more susceptible to apoptosis at 43.2 ± 18.0% compared to the younger group (<38 years old) with a mean percentage of apoptotic cells 33 ± 17.2%. Women who had a positive pregnancy had a lower level of apoptosis in GL cultures than those who did not get pregnant (30.2 ± 14% vs. 40.4 ± 19.5%). There was a positive correlation between embryo fragmentation and GL cell apoptosis ( r = 0.214). Overall, the level of apoptosis of cultured GL cells was correlated with IVF outcome [ 66 ].

These studies demonstrate the diversity among techniques to evaluate cell death in the developing embryo. TUNEL labeling, sperm comet assay, annexin V staining or some combination of these techniques have been described. Furthermore, there are discrepancies between the stage at which apoptosis might occur, with majority of studies cited here suggesting that cell death occurs in early stages of development before blastocyst formation. While some studies suggest that fragmented embryos display signs of apoptosis, these findings are still disputed and the distinction between apoptosis and necrosis is not clearly defined in the literature.

Patient age and CF

There are inconsistencies within the literature regarding the relationship between maternal age and CF. A total of six studies in this review focused on this relationship (Table 4 ). Three of the studies found a positive correlation between patient age and degree of embryo fragmentation [ 67 , 68 , 69 ]. The other three studies found no age-related correlation between embryo fragmentation or quality [ 7 , 70 , 71 ].

A retrospective study by Ziebe et al., compared the relationship between age of women undergoing IVF and the proportion of anucleate fragmentation in cleavage-stage embryos. Using a logistic regression analysis, the authors compared the percentage of transfers using fragmented embryos with age; the odds of fragmentation increased by 3% per year (OR 1.033 [95% CI 0.996, 1.071]). There was a linear relationship between age and embryo fragmentation rate, with an increase in fragmentation of 0.76% per year (95% CI -0.09%, 1.61%) [ 68 ].

Keltz et al., assessed various predictors of embryo fragmentation in IVF and found that increased maternal age and lower number of oocytes and embryos were associated with increased embryo fragmentation. There was a significant difference between cycles with fragmented embryos ( n =74) at a mean age of 36.9 ± 4.24 years as compared to cycles with no fragmented embryos ( n =234) at a mean age of 35.4 ± 4.74 years. Overall, this retrospective analysis of fresh IVF cycles found that embryo fragmentation is indeed associated with older age and ultimately poor cycle outcome [ 67 ].

Contrary to these findings, an early study by Alikani et al., showed no relationship between maternal age and CF [ 7 ]. In a retrospective analysis of degree and pattern of embryo fragmentation on days 2 and 3, they defined five patterns of fragmentation. Both the degree and pattern of fragmentation impacted pregnancy and implantation rate, but the authors found no correlation between appearance of any CF pattern and maternal age. The average maternal age in their population was 35.7 ± 4.25 years [ 7 ]. Another study by Stensen et al., analyzed the effect of chronological age on oocyte quality (assessed by maturity) and embryo quality (assessed by cleavage-stage, blastomere size and embryo fragmentation). Women were divided into five age groups: ≤25, 26–30, 31–35, 36–40 and ≥41 years. The embryo morphological score was based on fragmentation and blastomere size with score of 0-4 where score of 4 being equally sized blastomeres and no fragmentation and score of 0 being cleavage arrest or morphologically abnormal embryo. The mean oocyte score and embryo morphology score were not found to be significantly different across the age groups [ 70 ]. Wu et al., also showed that age does not influence embryo fragmentation. Patient ages ranged from 20 to 44 years with a mean age of 30.6 ± 4.6 years and were divided into age groups of ≤29, 30–34, 35–37, 38–40, and ≥41 years of age. Analysis of embryos with similar degrees of fragmentation was used to assess whether maternal age was associated with embryo fragmentation and blastocyst development. There was no correlation between age and embryo fragmentation as a continuous variable ( r = 0.02; P = 0.25) nor was there a correlation when age was divided into the groups ( P = 0.2). They also found that neither age ( r = -0.08; P =0.16) nor degree of fragmentation ( r = -0.01; P = 0.81) had a significant impact on blastocyst development [ 71 ].

Recently, a retrospective time-lapse study evaluated the implantation rate of 379 fragmented embryos. The results showed that there was an association between advanced maternal age and fragmentation. Fragmentation rate was higher in patients ˃35 compared to patients ≤35 years old. It seems that the lower quality of oocytes in older patients results in increasing fragmentation [ 69 ]. Overall, the included studies have differing conclusions on the effect of maternal age and CF; varying definitions and analysis of CF remain a limitation.

IVF vs ICSI procedures and CF

Five of the included studies compared embryo quality between conventional IVF and intracytoplasmic sperm injection (ICSI) procedures (Table 5 ). Two of these studies found that ICSI was associated with impaired embryo morphology compared to IVF [ 72 , 73 ], while the other three showed no difference in embryo quality between the two fertilization modalities [ 74 , 75 , 76 ]. There were no studies within our search that identified embryos created by ICSI having greater morphology grade, or less embryo fragmentation, than IVF.

Frattarelli et al., directly examined the effect of ICSI on embryo fragmentation and implantation rate compared to IVF. There was a significant difference in mean embryo grade between IVF and ICSI. IVF patients had significantly more grade I, or non-fragmented, embryos compared to the ICSI group ( P < 0.01). However, there was no significant difference in mean number of embryos per embryo grade II – IV [ 72 ].

Similarly, Hsu et al., compared embryo quality, morphology, and cleavage after ICSI with standard IVF patients. They defined the grading system from 1 – 5, ranging from no fragments (grade 1) to severe or complete fragmentation (grade 5). They found that for the overall population, when comparing ICSI and IVF patients after matching for age and number of embryos transferred, the number of embryos with good morphology was significantly greater in the IVF group compared to ICSI ( P < 0.006). The average morphology scores, similar to the results of Frattarelli et al., were significantly different between the ICSI group and the IVF group. They also found IVF patients’ embryos to have significantly better cleavage rate than those from ICSI patients ( P < 0.001) [ 73 ].

Garello et al., evaluated if fertilization via ICSI influences pronuclear orientation, PB placement, and embryo quality when compared to IVF. Embryos were assessed using morphology, and grouped as good (grades 1-2), average (grades 3-4), or poor (grades 5-6). Embryos were also assessed for cleavage regularity and proportion of fragmentation (0, <20%, 20–50%, >50%). There was no statistically significant difference in mean morphology (good, average, poor) between the groups, although they did note an apparent increase in grade 4 versus grade 3 embryos after ICSI procedure. The two groups had similar proportions of fragmentation [ 74 ].

Two other studies took a unique approach in comparing embryo quality in ICSI and IVF patients by using randomized sibling oocytes [ 75 , 76 ]. Yoeli et al., studied oocytes retrieved from patients with a less than 40% fertilization rate in a previous standard IVF cycle and divided these oocytes into a conventional insemination group and an ICSI group. Each group had over 1400 oocytes. Overall, there was no significant difference between the IVF and ICSI groups in terms of cleavage rate or rate of high-quality embryos (both Grade A embryos with ≤10% fragmentation and embryos with ≤20% fragmentation) [ 75 ]. Ruiz et al., also analyzed sibling oocytes in patients who had failed intrauterine insemination attempts. The authors similarly found no significant difference in fertilization rates and degree of fragmentation between ICSI and standard IVF groups [ 76 ]. Most studies included in the search criteria showed that ART techniques such as ICSI do not significantly impact fragmentation rate in developing embryos, suggesting that ICSI is not a significant contributor to poorer outcomes by way of embryo fragmentation. Of note, the timing of cumulus cell denudation after conventional IVF is a matter of debate; none of the included studies in this review performed short-time insemination. In a meta-analysis reviewing denudation times, the number of good quality embryos produced after retaining cumulus cells was similar to those produced after early removal of these cells, suggesting that brief insemination has no impact on CF [ 77 ]. Liu et al. also showed that short insemination time is not associated with different outcomes in terms of embryo development [ 78 ].

Effect of CF on embryo development

It is commonly believed that CF has detrimental effects on embryo development. Thirteen of the included studies found a negative effect of CF on embryo development (Table 6 ). Various approaches have been used to propose a hypothesis as to how increased fragmentation impedes embryo development.

Van Blerkom et al., showed through time-lapse video and TEM that fragments physically impede cell-cell interactions, interfering with compaction, cavitation, and blastocyst formation [ 63 ]. In an ultrastructural observational study by Sathananthan et al., 15 embryos were cultured with human ampullary cell lines and TEM used to evaluate embryo development. They noted degeneration of blastomeres, including incomplete incorporation of chromatin into nuclei and formation of micronuclei, which was possibly a consequence of being adjacent to blastomere fragments [ 79 ]. A much larger prospective study by Antczak and Van Blerkom analyzed 2293 fertilized eggs from 257 IVF cycles to examine the effect of fragmentation on the distribution of eight regulatory proteins. Fragmentation reduced the volume of cytoplasm and depleted embryos of essential organelles or regulatory proteins, compromising the embryo developmental potential. They also found that specific fragmentation patterns during various stages of embryo development, i.e., 2- and 4-cell stages, were associated with embryo viability and therefore could have clinical application in the selection of embryos for transfer [ 24 ]. As previously mentioned, fragmentation may affect compacted/morula and blastocyst quality [ 80 ]. Cell exclusion at this stage is due to failure or abnormal expression of proteins involved in compaction [ 44 , 81 ]. Blastomeres may also irregularly divide, resulting in fragmentation and exclusion from compaction [ 82 ], and excluded cells have a high rate of aneuploidy [ 83 ]. Blastocyst quality from fully compacted embryos has been reported to be higher than blastocysts with partial compaction [ 84 ].

The hypothesis that fragmentation reflects inherent embryogenetic abnormalities, such as aneuploidy, increased mosaicism, or polyploidy, is supported by multiple studies in this review [ 55 , 57 , 85 ]. Morphologically poor-quality embryos, defined by amount of fragmentation, were often found to have concomitant chromosomal abnormalities [ 57 , 85 ]. Culture environment has also been implicated in presence and degree of fragmentation. For example, Morgan et al., using video-cinematography found that embryos cultured on a monolayer of feeder cells had fewer fragments than did embryos cultured alone [ 86 ]. In addition to aneuploidy and external environment, degree of fragmentation also appears to be related to embryo quality. Both Alikani et al., and Hardy et al., have shown that a small degree of fragmentation (<15%) on day-2 embryos did not affect blastocyst formation but increased (> 15%) fragmentation was associated with significantly reduced blastocyst development [ 23 , 87 ]. Similarly, a prospective study of over 4000 embryos by Guerif et al., showed that the rate of blastocyst formation increased significantly with decreased fragmentation (<20%) on day-2 embryos [ 32 ].

A separate study by Ivec et al., graded day-4 and -5 morulae based on the degree of fragmentation (<5%, 5%–20%, or >20%) and compared their blastocyst development rate. They found a negative correlation between degree of fragmentation and clinically usable blastocysts, optimal blastocysts, and those with a hatching zona pellucida. Through logistic regression analysis, they found that with each increase in percentage of fragmentation in morulae, there was a 4% decrease in the odds of hatching (OR: 0.96, 95% CI: 0.95–0.98;  P < 0.001) and optimal blastocyst formation (OR: 0.96, 95% CI: 0.94–0.97;  P < 0.001) [ 88 ]. It is important to point out that the degree of embryo fragmentation, no matter at what stage of development, is measured subjectively without standardized methods. One study from Hnida et al., included here recognized this limitation and used a computer-controlled system for multilevel embryo morphology analysis [ 89 ]. The degree of fragmentation was evaluated based on digital image sequences and correlated to the blastomere size. Fragments were defined to be anucleate with an average diameter of <40 µm. Not surprisingly, the mean blastomere volume decreased significantly with increasing degree of fragmentation ( P < 0.001). In addition, average blastomere size was significantly affected by the degree of fragmentation and multinuclearity which may function as a biomarker for embryo quality [ 89 ]. Furthermore, Sjöblom et al., analyzed the relationship of morphological characteristics to the developmental potential of embryos [ 90 ]. These authors, similar to Hnida et al., found that a large cytoplasmic deficit, i.e., blastomeres not filling the space under the zona, was detrimental to blastocyst development (P < 0.044). However, this is the only study in which the extent of CF observed was not significantly associated with blastocyst development [ 90 ]. Another study using time-lapse imaging showed an association between cytoplasmic fragments at the two-cell stage and perivitelline threads. Perivitelline threads can be observed as the cytoplasmic membrane withdraws from the zona pellucida during embryo cleavage. Ultimately, the presence of these threads, despite the level of fragmentation, did not affect embryo development [ 91 ]. As demonstrated by the studies described here, the degree of CF has a largely negative effect on embryo development.

Effect of CF on embryo implantation and pregnancy

In addition to evaluating the effect of CF on preimplantation embryo development, it is important to assess the effect of CF on implantation and pregnancy outcomes. Five of the included studies have shown a negative effect of CF on implantation or pregnancy outcome (Table 7 ). Assuming that increased fragmentation is detrimental to embryo development, implantation, and pregnancy outcome, it is important to understand the embryo scoring system that determines the best embryo for transfer. Giorgetti et al., used single embryo transfers to devise an embryo scoring pattern to best predict successful implantation. Not surprisingly, higher pregnancy rates were observed with embryos that displayed no fragmentation. The authors found that both pregnancy rate and live birth rate were significantly correlated with a 4-point score based on cleavage rate, fragmentation, irregularities displayed, and presence of a 4-cell embryo on day-2 [ 12 ].

Racowsky et al., assessed if multiple evaluations of an embryo improve selection quality and thus implantation and pregnancy success. They noted that an increased level of fragmentation on both day-2 and -3 was associated with a significant reduction in the number of fetuses that developed to 12 weeks. They also noted that severe fragmentation (>50%) impaired overall embryo viability and may be related to low pregnancy rates and high risk of congenital malformations. The authors ultimately concluded that single day morphological evaluation on day-2 or day-3 has the same predictive value to a multi-day scoring system [ 22 ].

Another retrospective analysis of 460 fresh embryo transfers by Ebner et al., sought to determine the impact of embryo fragmentation on not just pregnancy, but also obstetric and perinatal outcomes. There was a significant relationship between fragmentation and implantation and clinical pregnancy rate, but not with multiple pregnancy rate or ongoing pregnancy rate [ 10 ]. Alikani et al., also studied embryo fragmentation and its implications for implantation and pregnancy rate and included fragmentation pattern into their discussion. They too found a significant decrease in implantation and pregnancy rate as the degree of fragmentation increased. They identified an effect on pregnancy rate when the degree of fragmentation was greater than 35%. The authors went on to discuss that not all fragmentations are detrimental to the embryo development and that the pattern of fragmentation matters. They found that fragmentation pattern type IV, defined as having large fragments distributed randomly and associated with uneven cells, had significantly lower implantation and clinical pregnancy rates when compared to types I-III. They concluded that detaching blastomere cytoplasm as large fragments is most detrimental to embryo development and implantation rate. In contrast, small, scattered fragments (type III) did not seem to appreciably affect the cell number or pose a serious threat to further development [ 7 ].

Lastly, Paternot et al., used sequential imaging techniques and a computer-assisted scoring system to study blastocyst development and the effect of fragmentation on clinical pregnancy. The authors reviewed the volume reduction over time as a measure of embryo fragmentation. They analyzed volumes on day-1 to -3 and found a significant association between total embryo volume and pregnancy rate on both day-2 ( P = 0.003) and day-3 ( P = 0.0003), with the total volume measured on day-3 being the best predictor of pregnancy outcome [ 92 ]. In contrast, Lahav-Baratz recently showed that there was no association between fragmentation rate and abortion or live birth rate. It was concluded that fragmented embryos still have implantation potential and could be considered for transfer when applicable [ 69 ].

Effect of CF removal on embryo development

The effect of fragment removal on IVF outcomes has been controversial. Six of the studies included in this review discussed the impact of removing fragments on embryo development (Table 8 ) [ 7 , 67 , 93 , 94 , 95 , 96 ]. The literature is mixed, with some studies showing improvement in embryo development quality after fragmentation removal [ 7 , 93 ], and others showing no difference at all [ 70 , 94 , 95 ].

Alikani et al., were one of the first investigators to define various patterns of fragmentation and perform microsurgical fragment removal to improve implantation potential [ 7 ]. The authors found that the pattern and degree of fragmentation, and not merely the presence of fragmentation, was significant. When assisted hatching and microsurgical fragment removal was performed, there was an overall 4% increase in implantation rate. They concluded that the removal of the fragments possibly restored the spatial relationship of the cells and limited the interference of cell-cell contact. Further, their preliminary data showed that blastocysts formed after fragment removal were better organized than their unmanipulated counterparts [ 7 ].

Eftekhari-Yazdi et al., similarly studied the effect of fragment removal on blastocyst formation and quality of embryos [ 93 ]. They compared day-2 embryos without removal of fragments to those that fragments were microsurgically removed. There were significantly higher quality embryos in defragmented group compared to the control. Furthermore, fragment removal improved the blastocyst quality compared to the control group. There was also a reduction of apoptotic and necrotic cells in experimental group when compared with the control group [ 93 ].

Two separate studies by Keltz et al., assessed implantation, clinical pregnancy, and birth outcomes after defragmentation [ 67 ], as well as embryo development and fragmentation rate after day-3 embryo defragmentation [ 94 ]. The authors first compared cycle outcomes between low-grade embryos that underwent micromanipulation for fragment removal (>10% fragmentation) and high-grade embryos that did not undergo defragmentation but were hatched on day 3. When compared, the defragmented group showed no difference in rates of implantation, clinical pregnancy, live birth, spontaneous abortion, or fetal defects as compared to the cycles that included all top-grade embryos. Factors associated with poor IVF prognosis and formation of embryo fragments included advanced age, decreased number of oocytes and embryos, and embryo grade [ 67 ].

A separate prospective randomized study by Keltz et al., looked more specifically at day-5 fragmentation, compaction, morulation and blastulation rates after low grade day-3 embryo defragmentation [ 94 ]. Paired embryos from the same patient, not intended to be transferred, were randomly placed in either the experimental group, assisted hatching and embryo defragmentation, or control group (assisted hatching alone). Paired embryos had no difference in mean cell number, percent fragmentation, and grade before randomization. Results showed that on day-5, embryos in the defragmentation group had significantly diminished fragmentation when compared with controls; however, there was no difference in compaction rate, morula formation rate or blastocyst formation rate. Embryo grade generally improved in the treatment group, but this was not statistically significant. Overall, in both groups, improved embryo development was significantly associated with lower levels of fragmentation in the day-3 embryos, supporting the idea that defragmented embryos maintain their reduced fragmented state throughout preimplantation development. Of note, this study had 35 embryos in each group and was limited to lower grade embryos not intended for transfer [ 94 ].

Another, larger prospective randomized study by Halvaei et al., compared the effect of microsurgical removal of fragments on ART outcomes. The authors divided 150 embryos with 10-50% fragmentation into three groups, case ( n =50), sham ( n =50), and control ( n =50). They found no significant difference in rates of clinical pregnancy, miscarriage, live birth, multiple pregnancies, or congenital anomalies between these groups, ultimately showing that cosmetic microsurgery on preimplantation embryos to remove CFs had no beneficial effect [ 95 ].

Lastly, a pilot study by Yumoto et al., aimed to decrease CF in developing embryos by removing the zona pellucida of abnormally fertilized (3PN) donated oocytes [ 96 ]. Although they did not attempt to remove fragments themselves, this study is included as ZP-free oocytes are sometimes encountered in or because of ART procedures, i.e., ICSI. The results suggest that the rate of fragmentation is decreased after mechanical ZP removal. The authors concluded that ZP is not always necessary for normal embryo development since the ZP-free embryos developed normally, maintained their cell adhesions, and had a decreased rate of fragmentation [ 96 ]. It seems that defragmentation of an aneuploid or severely fragmented embryo, only improves the embryo morphology grade but the quality and fate of embryo is not changed [ 97 ].

CF and chromosomal abnormalities in embryo

Although the relationship between DNA fragmentation and chromosomal abnormalities has been more commonly explored in the literature, CF may also be related to intrinsic chromosomal abnormalities in developing embryos. Fourteen studies included in this review explored this relationship (Table 9 ) [ 55 , 56 , 85 , 98 , 99 , 100 , 101 , 102 , 103 , 104 , 105 , 106 , 107 , 108 ].

CF was rarely seen in embryos with normal chromosomal content. Findikli et al., studied DNA fragmentation and aneuploidy in poor quality embryos by TUNEL and fluorescent in situ hybridization (FISH) techniques. Within seven chromosomally abnormal embryos, each had variable degrees of CF [ 98 ]. This study suggests that DNA fragmentation, being a sign of chromosomal abnormalities, may exist together with CF.

An earlier study by Munne et al., examined 524 embryos using FISH analysis for three to five chromosomes. While controlling for age, they divided the embryos into three groups: arrested, slow and/or fragmented, or morphologically and developmentally normal. They found that polyploidy was the most common chromosomal abnormality in the arrested embryo group and decreased with increasing embryonic competence, with 44.5% polyploidy in arrested compared to 2.1% in morphologically normal embryos. Maternal age was not associated with polyploidy rates, but aneuploidy significantly increased with maternal age in morphologically normal human embryos [ 57 ]. Another early study by Almeida and Bolton also examined the relationship between chromosomal abnormalities and embryonic developmental potential. They found that cleavage-stage embryos with poor morphology, defined as irregular shaped blastomeres with severe fragmentation, showed a higher incidence of chromosomal abnormalities than those with good morphology [ 100 ]. Magli et al., found a more direct relationship between chromosomal abnormalities and embryo fragmentation in a larger retrospective study of nearly 1600 embryos. There was a strong association between percentage of fragmentation and chromosomal abnormalities (monosomies and trisomies), where 90% of chromosomal abnormalities were found in embryos with greater than 40% fragmentation [ 101 ].

Another retrospective study comparing maternal age to embryo morphology and chromosomal abnormalities was conducted by Moayeri et al., By examining nine chromosomes in day-3 embryos, they found that morphology predicted chromosomal status in the advanced maternal age group (≥38 years old), but not in younger patients. Fragmentation alone predicted euploidy in both the advanced maternal age and younger groups. This suggests that cellular fragmentation may be a predictor of chromosomal competence and thus embryo developmental potential [ 102 ].

In contrast, Baltaci et al., examined 1,000 embryos and concluded that embryo morphology was not predictive of euploidy and that a considerable number of chromosomally abnormal embryos with good development potential may be selected for embryo transfer. They used FISH for five chromosomes and found that a large proportion of both normal and aneuploid embryos were evaluated as top quality (grade I). For example, 66% of chromosomally abnormal embryos were of good quality (grade I and II). They found no significant difference among aneuploid embryos when distributed by age. However, a higher embryo quality found in normal compared to aneuploid embryos [ 103 ].

In addition, Pellestor et al., compared the relationship between morphology and chromosomal abnormalities in two separate studies. The first study found that aneuploidy was the most frequently observed abnormality after cytogenetic analysis of preimplantation embryos [ 55 ]. They defined the quality of embryos as good (grade I and II) and poor (grades III and IV). There was an increased chromosomal abnormality in poor quality embryos (84.3%) when compared to embryos with good quality (33.9%). Both aneuploidy and fragmentation were shown to be predominant in poor quality embryos, whereas mosaicism and polyploidy were the most frequent abnormalities in good quality embryos [ 55 ]. Pellestor et al., also performed cytogenetic analysis on 411 poor-quality embryos (grade IV) [ 85 ]. Ninety percent of the successfully analyzed cases showed abnormal chromosome complements, with aneuploidy being the most frequently observed. These results further support that a large majority of poor grade embryos are chromosomally abnormal and ultimately offer low chance of reproductive success for either embryo transfer or cryopreservation [ 85 ].

A separate study by Chavez et al., combined time-lapse imaging with karyotypic status of blastomeres in the 4-cell embryo to test whether blastomere behavior may reflect chromosomal abnormalities, using array comparative genomic hybridization (aCGH), during early cleavage [ 56 ]. In time-lapse observations, a large proportion of aneuploid and triploid, but not euploid embryos, exhibited cellular fragmentation. They showed that the probability of aneuploidy increased with higher fragmentation and only 65% of the fragmented embryo would be expected to form blastocyst. Furthermore, all the aneuploid embryos with additional unbalanced sub-chromosomal errors exhibited CF. The authors concluded that although fragmentation alone at a single point in time does not predict embryo developmental potential, time-lapse imaging with dynamic fragmentation screening may help detect embryonic aneuploidy [ 56 ].

Two more recent studies also used aCGH to evaluate the association between embryo ploidy and fragmentation. Vera-Rodriguez et al., in a retrospective study, compared the rate of embryo aneuploidy between two groups of high (≥25%) and low (˂25%) fragmentation. They found that the rate of aneuploidy in high and low fragmentation was 62.5 and 46.3%, respectively. However, the difference was not statistically significant concluding that using degree of fragmentation alone is not suggested to predict the embryo ploidy status [ 107 ]. Minasi et al., in a case series evaluated 1730 blastocyst ploidy with aCGH. They showed that there is no significant difference between day-3 embryo morphology and embryo ploidy. However, the quality of blastocyst (inner cell mass grade, trophectoderm grade, degree of expansion) was associated with embryo ploidy [ 106 ].

In a recent meta-analysis, it was shown there is trend between degree of fragmentation and rate of aneuploidy [ 109 ]. A major source of controversy in both early and recent studies on aneuploidy and fragmentation is the variation in the methods and criteria used to evaluate these factors. One of the aspects that differ across studies include the technique for detecting aneuploidy; FISH vs aCGH. Recent studies have used aCGH to detect aneuploidy and found no clear relationship in this regard. Also, the quality of the matching between groups, the design of the study (retrospective vs prospective), the timing of the fragmentation assessment, the use of time-lapse imaging to monitor the fate of fragments are the other reasons for this discrepancy. There is still the lack of a clear cut-off point for the percentage of fragmentation to predict aneuploidy. Further powerful studies using new methods like next gene sequencing and tile-lapse systems are recommended to shed light on the relationship between fragmentation and aneuploidy.

The literature highlights that poor quality embryos have a higher incidence of chromosomal abnormalities. Notably, CF is rarely observed in embryos with normal chromosomal content. Technological advancements, such as TLM, offer promising avenues to enhance our understanding and detection of embryonic aneuploidy. Overall, these studies underscore the complexity of the relationship between fragmentation and chromosomal abnormalities, emphasizing the need for continued research to refine embryo selection strategies and improve reproductive outcomes.

Discussion and conclusion

The role of fragmentation in human embryo development and reproductive potential is widely recognized, albeit without standard definition nor agreed upon implication. While it has been shown that degree of fragmentation and embryo implantation potential are inversely proportional [ 5 , 7 , 9 , 10 , 11 , 12 , 13 , 14 , 15 , 16 , 17 , 18 , 19 , 20 , 21 ], the degree, pattern, and distribution of fragmentation as it relates to pregnancy outcome is debated in the literature. Our qualitative synthesis of 60 articles related to the study of embryo fragmentation and reproductive outcomes highlighted some of the challenges in analysis of fragmentation, while revealing trends in our evolving knowledge of how fragmentation may relate to functional development of the human embryo.

While fragmentation is best understood to be a natural process across species, the origin of fragmentation remains incompletely understood and likely multifactorial. Degree of fragmentation has been plausibly correlated to sperm DNA oxidation [ 37 ], errors in division [ 37 ], mitochondrial distribution [ 45 ], and overall embryo quality [ 39 ]. However, some causes of fragmentation are based on outdated studies and require validation in future research with higher quality and more advanced techniques. While cause of fragmentation remains a focus of investigation, advances in technology have allowed for more detailed analysis of its effect on embryo development and reproductive outcome. At the cellular level, increased fragmentation has been shown to be associated with higher rates of apoptosis, necrosis, and programmed cell death of cleavage-stage embryos [ 60 , 61 , 62 ]. Given the recognized significance of fragmentation on embryo development, it follows that many studies have been focused on IVF and ART impacts on fragmentation, as well as determining quantitative reproductive outcomes. In terms of other influences on degree of fragmentation, patient age was not universally found to be significantly associated with fragmentation [ 7 , 70 , 71 ] although age is certainly known to influence embryo quality. Most studies included in the search criteria showed that ART such as ICSI do not significantly impact fragmentation rate in developing embryos [ 74 , 75 , 76 ]. Those studies that found significant differences in embryo grading either between conventional fertilization and ICSI either did not find a difference in implantation or pregnancy rate or did not study it, suggesting that ICSI is not a significant contributor to poorer ART outcomes by way of embryo fragmentation.

In synthesizing the available data on ART and pregnancy outcomes with varying degrees of embryo fragmentation, most included studies did find a negative impact of increasing fragmentation on reproductive success while severe fragmentation does appear to be associated with poorer implantation rate and clinical pregnancy rate. This association may be related to the observation that increased fragmentation at the cleavage-stage embryo is related to chromosomal abnormalities incompatible with ongoing development or pregnancy.

The reviewed studies have several limitations. There are different grading systems in use that may impact detecting and reporting the degree of CF. Different criteria and terminology used in different studies may in turn make the comparison of outcome measures difficult. Another factor is the distribution pattern of CF. There are two types of scattered and concentrated fragments with different prognoses that is not considered in grading systems. Therefore, due to the lack of a standard cleavage-stage embryo grading system, comparing different studies should be done with caution. In addition, evaluation of embryo fragmentation is mostly based on individual observation which is subjective and has inter- and intra-observer subjectivity leading to high variable results even if performed by an experienced embryologist [ 110 ]. TLM is considered as a non-invasive tool and evaluates the embryo quality continuously and without the need to remove the embryo from the incubator [ 111 ]. The use of this technology allows for the analysis of embryo morphokinetics and has advanced knowledge of the developing embryo. Recently, artificial intelligence (AI) including machine learning and neural network has gained popularity in various fields of medicine including IVF and embryology. Accuracy of AI in prediction of fragmentation has been studied with encouraging results [ 112 ]. Further advances in technology will promote the use of AI as a tool in defining the effect of fragmentation on human embryo development and reproductive potential.

Although the precise origin and the importance of external or iatrogenic factors on fragmentation of cleavage-stage embryos varies in the literature, there is more consensus regarding severe fragmentation worsening reproductive outcomes. Given this important pattern, and the availability of increasingly sophisticated embryologic technology, further research is warranted to characterize more completely preventative or rescue techniques to improve reproductive outcomes.

Availability of data and materials

No datasets were generated or analysed during the current study.

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Yazdani, A., Halvaei, I., Boniface, C. et al. Effect of cytoplasmic fragmentation on embryo development, quality, and pregnancy outcome: a systematic review of the literature. Reprod Biol Endocrinol 22 , 55 (2024). https://doi.org/10.1186/s12958-024-01217-7

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Received : 20 November 2023

Accepted : 01 April 2024

Published : 14 May 2024

DOI : https://doi.org/10.1186/s12958-024-01217-7

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