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• Steps in Conducting a Literature Review

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.

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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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Literature Reviews

Steps in the literature review process.

• What is a literature review?
• Define your research question
• Determine inclusion and exclusion criteria
• Choose databases and search
• Review Results
• Synthesize Results
• Analyze Results
• Librarian Support
• You may need to some exploratory searching of the literature to get a sense of scope, to determine whether you need to narrow or broaden your focus
• Identify databases that provide the most relevant sources, and identify relevant terms (controlled vocabularies) to add to your search strategy
• Finalize your research question
• Think about relevant dates, geographies (and languages), methods, and conflicting points of view
• Conduct searches in the published literature via the identified databases
• Check to see if this topic has been covered in other discipline's databases
• Examine the citations of on-point articles for keywords, authors, and previous research (via references) and cited reference searching.
• Save your search results in a citation management tool (such as Zotero, Mendeley or EndNote)
• De-duplicate your search results
• Make sure that you've found the seminal pieces -- they have been cited many times, and their work is considered foundational 
• Check with your professor or a librarian to make sure your search has been comprehensive
• Evaluate the strengths and weaknesses of individual sources and evaluate for bias, methodologies, and thoroughness
• Group your results in to an organizational structure that will support why your research needs to be done, or that provides the answer to your research question  
• Develop your conclusions
• Are there gaps in the literature?
• Where has significant research taken place, and who has done it?
• Is there consensus or debate on this topic?
• Which methodological approaches work best?
• For example: Background, Current Practices, Critics and Proponents, Where/How this study will fit in 
• Organize your citations and focus on your research question and pertinent studies
• Compile your bibliography

Note: The first four steps are the best points at which to contact a librarian. Your librarian can help you determine the best databases to use for your topic, assess scope, and formulate a search strategy.

Videos Tutorials about Literature Reviews

This 4.5 minute video from Academic Education Materials has a Creative Commons License and a British narrator.

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• What is a Literature Review? | Guide, Template, & Examples

What is a Literature Review? | Guide, Template, & Examples

Published on 22 February 2022 by Shona McCombes . Revised on 7 June 2022.

What is a literature review? A literature review is a survey of scholarly sources on a specific topic. It provides an overview of current knowledge, allowing you to identify relevant theories, methods, and gaps in the existing research.

There are five key steps to writing a literature review:

• Search for relevant literature
• Evaluate sources
• Identify themes, debates and gaps
• Outline the structure
• Write your literature review

A good literature review doesn’t just summarise sources – it analyses, synthesises, and critically evaluates to give a clear picture of the state of knowledge on the subject.

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Table of contents

Why write a literature review, examples of literature reviews, step 1: search for relevant literature, step 2: evaluate and select sources, step 3: identify themes, debates and gaps, step 4: outline your literature review’s structure, step 5: write your literature review, frequently asked questions about literature reviews, introduction.

• Quick Run-through
• Step 1 & 2

When you write a dissertation or thesis, you will have to conduct a literature review to situate your research within existing knowledge. The literature review gives you a chance to:

• Demonstrate your familiarity with the topic and scholarly context
• Develop a theoretical framework and methodology for your research
• Position yourself in relation to other researchers and theorists
• Show how your dissertation addresses a gap or contributes to a debate

You might also have to write a literature review as a stand-alone assignment. In this case, the purpose is to evaluate the current state of research and demonstrate your knowledge of scholarly debates around a topic.

The content will look slightly different in each case, but the process of conducting a literature review follows the same steps. We’ve written a step-by-step guide that you can follow below.

Prevent plagiarism, run a free check.

Writing literature reviews can be quite challenging! A good starting point could be to look at some examples, depending on what kind of literature review you’d like to write.

• Example literature review #1: “Why Do People Migrate? A Review of the Theoretical Literature” ( Theoretical literature review about the development of economic migration theory from the 1950s to today.)
• Example literature review #2: “Literature review as a research methodology: An overview and guidelines” ( Methodological literature review about interdisciplinary knowledge acquisition and production.)
• Example literature review #3: “The Use of Technology in English Language Learning: A Literature Review” ( Thematic literature review about the effects of technology on language acquisition.)
• Example literature review #4: “Learners’ Listening Comprehension Difficulties in English Language Learning: A Literature Review” ( Chronological literature review about how the concept of listening skills has changed over time.)

You can also check out our templates with literature review examples and sample outlines at the links below.

Download Word doc Download Google doc

Before you begin searching for literature, you need a clearly defined topic .

If you are writing the literature review section of a dissertation or research paper, you will search for literature related to your research objectives and questions .

If you are writing a literature review as a stand-alone assignment, you will have to choose a focus and develop a central question to direct your search. Unlike a dissertation research question, this question has to be answerable without collecting original data. You should be able to answer it based only on a review of existing publications.

Make a list of keywords

Start by creating a list of keywords related to your research topic. Include each of the key concepts or variables you’re interested in, and list any synonyms and related terms. You can add to this list if you discover new keywords in the process of your literature search.

• Social media, Facebook, Instagram, Twitter, Snapchat, TikTok
• Body image, self-perception, self-esteem, mental health
• Generation Z, teenagers, adolescents, youth

Search for relevant sources

Use your keywords to begin searching for sources. Some databases to search for journals and articles include:

• Your university’s library catalogue
• Google Scholar
• Project Muse (humanities and social sciences)
• Medline (life sciences and biomedicine)
• EconLit (economics)
• Inspec (physics, engineering and computer science)

You can use boolean operators to help narrow down your search:

Read the abstract to find out whether an article is relevant to your question. When you find a useful book or article, you can check the bibliography to find other relevant sources.

To identify the most important publications on your topic, take note of recurring citations. If the same authors, books or articles keep appearing in your reading, make sure to seek them out.

You probably won’t be able to read absolutely everything that has been written on the topic – you’ll have to evaluate which sources are most relevant to your questions.

For each publication, ask yourself:

• What question or problem is the author addressing?
• What are the key concepts and how are they defined?
• What are the key theories, models and methods? Does the research use established frameworks or take an innovative approach?
• What are the results and conclusions of the study?
• How does the publication relate to other literature in the field? Does it confirm, add to, or challenge established knowledge?
• How does the publication contribute to your understanding of the topic? What are its key insights and arguments?
• What are the strengths and weaknesses of the research?

Make sure the sources you use are credible, and make sure you read any landmark studies and major theories in your field of research.

You can find out how many times an article has been cited on Google Scholar – a high citation count means the article has been influential in the field, and should certainly be included in your literature review.

The scope of your review will depend on your topic and discipline: in the sciences you usually only review recent literature, but in the humanities you might take a long historical perspective (for example, to trace how a concept has changed in meaning over time).

Remember that you can use our template to summarise and evaluate sources you’re thinking about using!

Take notes and cite your sources

As you read, you should also begin the writing process. Take notes that you can later incorporate into the text of your literature review.

It’s important to keep track of your sources with references to avoid plagiarism . It can be helpful to make an annotated bibliography, where you compile full reference information and write a paragraph of summary and analysis for each source. This helps you remember what you read and saves time later in the process.

You can use our free APA Reference Generator for quick, correct, consistent citations.

To begin organising your literature review’s argument and structure, you need to understand the connections and relationships between the sources you’ve read. Based on your reading and notes, you can look for:

• Trends and patterns (in theory, method or results): do certain approaches become more or less popular over time?
• Themes: what questions or concepts recur across the literature?
• Debates, conflicts and contradictions: where do sources disagree?
• Pivotal publications: are there any influential theories or studies that changed the direction of the field?
• Gaps: what is missing from the literature? Are there weaknesses that need to be addressed?

This step will help you work out the structure of your literature review and (if applicable) show how your own research will contribute to existing knowledge.

• Most research has focused on young women.
• There is an increasing interest in the visual aspects of social media.
• But there is still a lack of robust research on highly-visual platforms like Instagram and Snapchat – this is a gap that you could address in your own research.

There are various approaches to organising the body of a literature review. You should have a rough idea of your strategy before you start writing.

Depending on the length of your literature review, you can combine several of these strategies (for example, your overall structure might be thematic, but each theme is discussed chronologically).

Chronological

The simplest approach is to trace the development of the topic over time. However, if you choose this strategy, be careful to avoid simply listing and summarising sources in order.

Try to analyse patterns, turning points and key debates that have shaped the direction of the field. Give your interpretation of how and why certain developments occurred.

If you have found some recurring central themes, you can organise your literature review into subsections that address different aspects of the topic.

For example, if you are reviewing literature about inequalities in migrant health outcomes, key themes might include healthcare policy, language barriers, cultural attitudes, legal status, and economic access.

Methodological

If you draw your sources from different disciplines or fields that use a variety of research methods , you might want to compare the results and conclusions that emerge from different approaches. For example:

• Look at what results have emerged in qualitative versus quantitative research
• Discuss how the topic has been approached by empirical versus theoretical scholarship
• Divide the literature into sociological, historical, and cultural sources

Theoretical

A literature review is often the foundation for a theoretical framework . You can use it to discuss various theories, models, and definitions of key concepts.

You might argue for the relevance of a specific theoretical approach, or combine various theoretical concepts to create a framework for your research.

Like any other academic text, your literature review should have an introduction , a main body, and a conclusion . What you include in each depends on the objective of your literature review.

The introduction should clearly establish the focus and purpose of the literature review.

If you are writing the literature review as part of your dissertation or thesis, reiterate your central problem or research question and give a brief summary of the scholarly context. You can emphasise the timeliness of the topic (“many recent studies have focused on the problem of x”) or highlight a gap in the literature (“while there has been much research on x, few researchers have taken y into consideration”).

Depending on the length of your literature review, you might want to divide the body into subsections. You can use a subheading for each theme, time period, or methodological approach.

As you write, make sure to follow these tips:

• Summarise and synthesise: give an overview of the main points of each source and combine them into a coherent whole.
• Analyse and interpret: don’t just paraphrase other researchers – add your own interpretations, 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 transitions and topic sentences to draw connections, comparisons and contrasts.

In the conclusion, you should summarise the key findings you have taken from the literature and emphasise their significance.

If the literature review is part of your dissertation or thesis, reiterate how your research addresses gaps and contributes new knowledge, or discuss how you have drawn on existing theories and methods to build a framework for your research. This can lead directly into your methodology section.

A literature review is a survey of scholarly sources (such as books, journal articles, and theses) related to a specific topic or research question .

It is often written as part of a dissertation , thesis, research paper , or proposal .

There are several reasons to conduct a literature review at the beginning of a research project:

• To familiarise yourself with the current state of knowledge on your topic
• To ensure that you’re not just repeating what others have already done
• To identify gaps in knowledge and unresolved problems that your research can address
• To develop your theoretical framework and methodology
• To provide an overview of the key findings and debates on the topic

Writing the literature review shows your reader how your work relates to existing research and what new insights it will contribute.

The literature review usually comes near the beginning of your  dissertation . After the introduction , it grounds your research in a scholarly field and leads directly to your theoretical framework or methodology .

Cite this Scribbr article

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McCombes, S. (2022, June 07). What is a Literature Review? | Guide, Template, & Examples. Scribbr. Retrieved 20 May 2024, from https://www.scribbr.co.uk/thesis-dissertation/literature-review/

Is this article helpful?

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Research Process :: Step by Step

• Introduction
• Select Topic
• Identify Keywords
• Background Information
• Develop Research Questions
• Refine Topic
• Search Strategy
• Popular Databases
• Evaluate Sources
• Types of Periodicals
• Reading Scholarly Articles
• Primary & Secondary Sources
• Organize / Take Notes
• Writing & Grammar Resources
• Annotated Bibliography
• Literature Review
• Citation Styles
• Paraphrasing
• Privacy / Confidentiality
• Research Process
• Selecting Your Topic
• Identifying Keywords
• Gathering Background Info
• Evaluating Sources

Organize the literature review into sections that present themes or identify trends, including relevant theory. You are not trying to list all the material published, but to synthesize and evaluate it according to the guiding concept of your thesis or research question.  

What is a literature review?

A literature review is an account of what has been published on a topic by accredited scholars and researchers. Occasionally you will be asked to write one as a separate assignment, but more often it is part of the introduction to an essay, research report, or thesis. 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

A literature review must do these things:

• be organized around and related directly to the thesis or research question you are developing
• synthesize results into a summary of what is and is not known
• identify areas of controversy in the literature
• formulate questions that need further research

Ask yourself questions like these:

• What is the specific thesis, problem, or research question that my literature review helps to define?
• What type of literature review am I conducting? Am I looking at issues of theory? methodology? policy? quantitative research (e.g. on the effectiveness of a new procedure)? qualitative research (e.g., studies of loneliness among migrant workers)?
• What is the scope of my literature review? What types of publications am I using (e.g., journals, books, government documents, popular media)? What discipline am I working in (e.g., nursing psychology, sociology, medicine)?
• How good was my information seeking? Has my search been wide enough to ensure I've found all the relevant material? Has it been narrow enough to exclude irrelevant material? Is the number of sources I've used appropriate for the length of my paper?
• Have I critically analyzed the literature I use? Do I follow through a set of concepts and questions, comparing items to each other in the ways they deal with them? Instead of just listing and summarizing items, do I assess them, discussing strengths and weaknesses?
• Have I cited and discussed studies contrary to my perspective?
• Will the reader find my literature review relevant, appropriate, and useful?

Ask yourself questions like these about each book or article you include:

• Has the author formulated a problem/issue?
• Is it clearly defined? Is its significance (scope, severity, relevance) clearly established?
• Could the problem have been approached more effectively from another perspective?
• What is the author's research orientation (e.g., interpretive, critical science, combination)?
• What is the author's theoretical framework (e.g., psychological, developmental, feminist)?
• What is the relationship between the theoretical and research perspectives?
• Has the author evaluated the literature relevant to the problem/issue? Does the author include literature taking positions she or he does not agree with?
• In a research study, how good are the basic components of the study design (e.g., population, intervention, outcome)? How accurate and valid are the measurements? Is the analysis of the data accurate and relevant to the research question? Are the conclusions validly based upon the data and analysis?
• In material written for a popular readership, does the author use appeals to emotion, one-sided examples, or rhetorically-charged language and tone? Is there an objective basis to the reasoning, or is the author merely "proving" what he or she already believes?
• How does the author structure the argument? Can you "deconstruct" the flow of the argument to see whether or where it breaks down logically (e.g., in establishing cause-effect relationships)?
• In what ways does this book or article contribute to our understanding of the problem under study, and in what ways is it useful for practice? What are the strengths and limitations?
• How does this book or article relate to the specific thesis or question I am developing?

Text written by Dena Taylor, Health Sciences Writing Centre, University of Toronto

http://www.writing.utoronto.ca/advice/specific-types-of-writing/literature-review

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• 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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Literature Review: Conducting & Writing

• Steps for Conducting a Lit Review

1. Choose a topic. Define your research question.

2. decide on the scope of your review., 3. select the databases you will use to conduct your searches., 4. conduct your searches and find the literature. keep track of your searches, 5. review the literature..

• Finding "The Literature"
• Organizing/Writing
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• Chicago: Notes Bibliography This link opens in a new window
• MLA Style This link opens in a new window
• Sample Literature Reviews

Disclaimer!!

Conducting a literature review is usually recursive, meaning that somewhere along the way, you'll find yourself repeating steps out-of-order.

That is actually a good sign.  

Reviewing the research should lead to more research questions and those questions will likely lead you to either revise your initial research question or go back and find more literature related to a more specific aspect of your research question.

Your literature review should be guided by a central research question.  Remember, it is not a collection of loosely related studies in a field but instead 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.

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

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

Make a list of the databases you will search.  Remember to include comprehensive databases such as WorldCat and Dissertations & Theses, if you need to.

Where to find databases:

• Find Databases by Subject UWF Databases categorized by discipline
• Find Databases via Research Guides Librarians create research guides for all of the disciplines on campus! Take advantage of their expertise and see what discipline-specific search strategies they recommend!
• Review the abstracts of research studies carefully. This will save you time.
• Write down the searches you conduct in each database so that you may duplicate them if you need to later (or avoid dead-end searches   that you'd forgotten you'd already tried).
• Use the bibliographies and references of research studies you find to locate others.
• Ask your professor or a scholar in the field if you are missing any key works in the field.
• Use RefWorks to keep track of your research citations. See the RefWorks Tutorial if you need help.

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: 

• Again, review the abstracts carefully.  
• Keep careful notes so that you may track your thought processes during the research process.
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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.

Write a Literature Review

1. narrow your topic and select papers accordingly, 2. search for literature, 3. read the selected articles thoroughly and evaluate them, 4. organize the selected papers by looking for patterns and by developing subtopics, 5. develop a thesis or purpose statement, 6. write the paper, 7. review your work.

• Resources for Gathering and Reading the Literature
• Resources for Writing and Revising
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Consider your specific area of study. Think about what interests you and what interests other researchers in your field.

Talk to your professor, brainstorm, and read lecture notes and recent issues of periodicals in the field.

Limit your scope to a smaller topic area (ie. focusing on France's role in WWII instead of focusing on WWII in general).

• Four Steps to Narrow Your Research Topic (Video) This 3-minute video provides instructions on how to narrow the focus of your research topic.
• Developing a Research Question + Worksheet Use this worksheet to develop, assess, and refine your research questions. There is also a downloadable PDF version.

Define your source selection criteria (ie. articles published between a specific date range, focusing on a specific geographic region, or using a specific methodology).

Using keywords, search a library database.

Reference lists of recent articles and reviews can lead to other useful papers.

Include any studies contrary to your point of view.

Evaluate and synthesize the studies' findings and conclusions.

Note the following:

• Assumptions some or most researchers seem to make
• Methodologies, testing procedures, subjects, material tested researchers use
• Experts in the field: names/labs that are frequently referenced
• Conflicting theories, results, methodologies
• Popularity of theories and how this has/has not changed over time
• Findings that are common/contested
• Important trends in the research
• The most influential theories

Tip: If your literature review is extensive, find a large table surface, and on it place post-it notes or filing cards to organize all your findings into categories.

• Move them around if you decide that (a) they fit better under different headings, or (b) you need to establish new topic headings.
• Develop headings/subheadings that reflect the major themes and patterns you detected

Write a one or two sentence statement summarizing the conclusion you have reached about the major trends and developments you see in the research that has been conducted on your subject.

• Templates for Writing Thesis Statements This template provides a two-step guide for writing thesis statements. There is also a downloadable PDF version.
• 5 Types of Thesis Statements Learn about five different types of thesis statements to help you choose the best type for your research. There is also a downloadable PDF version.
• 5 Questions to Strengthen Your Thesis Statement Follow these five steps to strengthen your thesis statements. There is also a downloadable PDF version.

Follow the organizational structure you developed above, including the headings and subheadings you constructed.

Make certain that each section links logically to the one before and after.

Structure your sections by themes or subtopics, not by individual theorists or researchers.

• Tip: If you find that each paragraph begins with a researcher's name, it might indicate that, instead of evaluating and comparing the research literature from an analytical point of view, you have simply described what research has been done.

Prioritize analysis over description.

• For example, look at the following two passages and note that Student A merely describes the literature, whereas Student B takes a more analytical and evaluative approach by comparing and contrasting. You can also see that this evaluative approach is well signaled by linguistic markers indicating logical connections (words such as "however," "moreover") and phrases such as "substantiates the claim that," which indicate supporting evidence and Student B's ability to synthesize knowledge.

Student A: Smith (2000) concludes that personal privacy in their living quarters is the most important factor in nursing home residents' perception of their autonomy. He suggests that the physical environment in the more public spaces of the building did not have much impact on their perceptions. Neither the layout of the building nor the activities available seem to make much difference. Jones and Johnstone make the claim that the need to control one's environment is a fundamental need of life (2001), and suggest that the approach of most institutions, which is to provide total care, may be as bad as no care at all. If people have no choices or think that they have none, they become depressed.

Student B: After studying residents and staff from two intermediate care facilities in Calgary, Alberta, Smith (2000) came to the conclusion that except for the amount of personal privacy available to residents, the physical environment of these institutions had minimal if any effect on their perceptions of control (autonomy). However, French (1998) and Haroon (2000) found that availability of private areas is not the only aspect of the physical environment that determines residents' autonomy. Haroon interviewed 115 residents from 32 different nursing homes known to have different levels of autonomy (2000). It was found that physical structures, such as standardized furniture, heating that could not be individually regulated, and no possession of a house key for residents limited their feelings of independence. Moreover, Hope (2002), who interviewed 225 residents from various nursing homes, substantiates the claim that characteristics of the institutional environment such as the extent of resources in the facility, as well as its location, are features which residents have indicated as being of great importance to their independence.

• How to Integrate Critical Voice into Your Literature Review (Video)
• Look at the topic sentences of each paragraph. If you were to read only these sentences, would you find that your paper presented a clear position, logically developed, from beginning to end? The topic sentences of each paragraph should indicate the main points of your literature review.
• Make an outline of each section of the paper and decide whether you need to add information, to delete irrelevant information, or to re-structure sections.
• Read your work out loud. That way you will be better able to identify where you need punctuation marks to signal pauses or divisions within sentences, where you have made grammatical errors, or where your sentences are unclear.
• Since the purpose of a literature review is to demonstrate that the writer is familiar with the important professional literature on the chosen subject, check to make certain that you have covered all of the important, up-to-date, and pertinent texts. In the sciences and some of the social sciences it is important that your literature be quite recent; this is not so important in the humanities.
• Make certain that all of the citations and references are correct and that you are referencing in the appropriate style for your discipline. If you are uncertain which style to use, ask your professor.
• Check to make sure that you have not plagiarized either by failing to cite a source of information, or by using words quoted directly from a source. (Usually if you take three or more words directly from another source, you should put those words within quotation marks, and cite the page.)
• Text should be written in a clear and concise academic style; it should not be descriptive in nature or use the language of everyday speech.
• There should be no grammatical or spelling errors.
• Sentences should flow smoothly and logically.
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Conduct a literature review

What is a literature review.

A literature review is a summary of the published work in a field of study. This can be a section of a larger paper or article, or can be the focus of an entire paper. Literature reviews show that you have examined the breadth of knowledge and can justify your thesis or research questions. They are also valuable tools for other researchers who need to find a summary of that field of knowledge.

Unlike an annotated bibliography, which is a list of sources with short descriptions, a literature review synthesizes sources into a summary that has a thesis or statement of purpose—stated or implied—at its core.

How do I write a literature review?

Step 1: define your research scope.

• What is the specific research question that your literature review helps to define?
• Are there a maximum or minimum number of sources that your review should include?

Ask us if you have questions about refining your topic, search methods, writing tips, or citation management.

Step 2: Identify the literature

Start by searching broadly. Literature for your review will typically be acquired through scholarly books, journal articles, and/or dissertations. Develop an understanding of what is out there, what terms are accurate and helpful, etc., and keep track of all of it with citation management tools . If you need help figuring out key terms and where to search, ask us .

Use citation searching to track how scholars interact with, and build upon, previous research:

• Mine the references cited section of each relevant source for additional key sources
• Use Google Scholar or Scopus to find other sources that have cited a particular work

Step 3: Critically analyze the literature

Key to your literature review is a critical analysis of the literature collected around your topic. The analysis will explore relationships, major themes, and any critical gaps in the research expressed in the work. Read and summarize each source with an eye toward analyzing authority, currency, coverage, methodology, and relationship to other works. The University of Toronto's Writing Center provides a comprehensive list of questions you can use to analyze your sources.

Step 4: Categorize your resources

Divide the available resources that pertain to your research into categories reflecting their roles in addressing your research question. Possible ways to categorize resources include organization by:

• methodology
• theoretical/philosophical approach

Regardless of the division, each category should be accompanied by thorough discussions and explanations of strengths and weaknesses, value to the overall survey, and comparisons with similar sources. You may have enough resources when:

• You've used multiple databases and other resources (web portals, repositories, etc.) to get a variety of perspectives on the research topic.
• The same citations are showing up in a variety of databases.

Additional resources

Undergraduate student resources.

• Literature Review Handout (University of North Carolina at Chapel Hill)
• Learn how to write a review of literature (University of Wisconsin-Madison)

Graduate student and faculty resources

• Information Research Strategies (University of Arizona)
• Literature Reviews: An Overview for Graduate Students (NC State University)
• Oliver, P. (2012). Succeeding with Your Literature Review: A Handbook for Students [ebook]
• Machi, L. A. & McEvoy, B. T. (2016). The Literature Review: Six Steps to Success
• Graustein, J. S. (2012). How to Write an Exceptional Thesis or Dissertation: A Step-by-Step Guide from Proposal to Successful Defense [ebook]
• Thomas, R. M. & Brubaker, D. L. (2008). Theses and Dissertations: A Guide to Planning, Research, and Writing

Literature Review

• Steps for Conducting a Lit Review
• Finding "The Literature"
• Organizing/Writing
• Sample Literature Reviews
• FAMU Writing Center

1. Choose a topic. Define your research question.

Your literature review should be guided by a central research question.  Remember, it is not a collection of loosely related studies in a field but instead 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.

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? 

Tip: 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. 

• Look at the Library's research guides in your discipline to select discipline-specific databases.  Don't forget to look at books!
• Make an appointment with or contact your   subject librarian to make sure you aren't missing major databases.

4. Conduct your searches and find the literature. Keep track of your searches!

Tips: 

• Review the abstracts of research studies carefully. This will save you time.
• Write down the searches you conduct in each database so that you may duplicate them if you need to later (or avoid dead-end searches   that you'd forgotten you'd already tried).
• Use the bibliographies and references of research studies you find to locate others.
• Ask your professor or a scholar in the field if you are missing any key works in the field.
• Use RefWorks to keep track of your research citations. See the RefWorks Tutorial if you need help.

5. 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?
• Again, review the abstracts carefully.  
• Keep careful notes so that you may track your thought processes during the research process.

Composing your literature review

O nce you've settled on a general pattern of organization, you're ready to write each section. There are a few guidelines you should follow during the writing stage. Here is a sample paragraph from a literature review about sexism and language to illuminate the following discussion:

  However, other studies have shown that even gender-neutral antecedents are more likely to produce masculine images than feminine ones (Gastil, 1990). Hamilton (1988) asked students to complete sentences that required them to fill in pronouns that agreed with gender-neutral antecedents such as "writer," "pedestrian," and "persons." The students were asked to describe any image they had when writing the sentence. Hamilton found that people imagined 3.3 men to each woman in the masculine "generic" condition and 1.5 men per woman in the unbiased condition. Thus, while ambient sexism accounted for some of the masculine bias, sexist language amplified the effect. (Source: Erika Falk and Jordan Mills, "Why Sexist Language Affects Persuasion: The Role of Homophily, Intended Audience, and Offense," Women and Language19:2.

Use evidence

In the example above, the writers refer to several other sources when making their point. A literature review in this sense is just like any other academic research paper. Your interpretation of the available sources must be backed up with evidence to show 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 review's focus, whether it is thematic, methodological, or chronological.

Use quotes sparingly

Falk and Mills do not use any direct quotes. That is because the survey nature of the literature review does not allow for in-depth discussion or detailed quotes from the text. Some short quotes here and there are okay, though if you want to emphasize a point, or if what the author said just cannot be rewritten in your own words. Notice that Falk and Mills do quote certain terms that were coined by the author, not common knowledge, or taken directly from the study. But if you find yourself wanting to put in more quotes, check with your instructor.

Summarize and synthesize

Remember to summarize and synthesize your sources within each paragraph as well as throughout the review. The authors here recapitulate important features of Hamilton's study, but then synthesize it by rephrasing the study's significance and relating it to their own work.

Keep your own voice

While the literature review presents others' ideas, your voice (the writer's) should remain front and center. Notice that Falk and Mills weave references to other sources into their own text, but they still maintain their own voice by starting and ending the paragraph with their own ideas and their own words. The sources support what Falk and Mills are saying.

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. In the preceding example, Falk and Mills either directly refer in the text to the author of their source, such as Hamilton, or they provide ample notation in the text when the ideas they are mentioning are not their own, for example, Gastil's. For more information, please see our handout on plagiarism .

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Conducting a Literature Review

Steps in conducting a literature review.

• Benefits of 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

Conducting a literature review involves using research databases to identify materials that cover or are related in some sense to the research topic. In some cases the research topic may be so original in its scope that no one has done anything exactly like it, so research that is at least similar or related will provide source material for the literature review. The selection of databases will be driven by the subject matter and the scope of the project.

Selecting Databases -- Most academic libraries now provide access to a majority of their databases and their catalog via a so-called discovery tool. A discovery tool makes searching library systems more "Google-like" in that even the simplest of queries can be entered and results retrieved. However, many times the results are also "Google-like" in the sheer quantity of items retrieved. While a discovery tool can be invaluable for quickly finding a multitude of resources on nearly any topic, there are a number of considerations a researcher should keep in mind when using a discovery tool, especially for the researcher who is attempting a comprehensive literature review.

No discovery tool works with every database subscribed to by a library. Some libraries might subscribe to two or three hundred different research databases covering a large number of subject areas. Competing discovery systems might negotiate agreements with different database vendors in order to provide access to a large range of materials. There will be other vendors with whom agreements are not forthcoming, therefore their materials are not included in the discovery tool results. While this might be of only minor concern for a researcher looking to do a fairly limited research project, the researcher looking to do a comprehensive review of the literature in preparation for writing a master's thesis or a doctoral dissertation will run the risk of missing some materials by limiting the search just to a particular library's discovery system. If only one system covered everything that a researcher could possibly need, libraries would have no need to subscribe to hundreds of different databases. The reality is that no one tool does it all. Not even Google Scholar.

Book collections might be excluded from results delivered by a discovery tool. While many libraries are making results from their own catalogs available via their discovery tools, they might not cover books that are discoverable from other library collections, thus making a search of book collections incomplete. Most libraries subscribe to an international database of library catalogs known as WorldCat. This database will provide comprehensive coverage of books, media, and other physical library materials available in libraries worldwide.

Features available in a particular database might not be available in a discovery tool. Keep in mind that a discovery tool is a search system that enables searching across content from numerous individual databases. An individual database might have search features that cannot be provided through a discovery tool, since the discovery tool is designed to accommodate a large number of systems with a single search. For example, the nursing database  CINAHL  includes the ability to limit a search to specific practice areas, to limit to evidence-based practice, to limit to gender, and to search using medical subject headings, among other things, all specialized facets that are not available in a discovery tool. To have these advanced capabilities, a researcher would need to go directly to  CINAHL  and search it natively.

Some discovery tools are set, by default, to limit search results to those items directly available through a particular library's collections. While many researchers will be most concerned with what is immediately available to them at their own library, a researcher concerned with finding everything that has been done on a particular topic will need to go beyond what's available at his or her home library and include materials that are available elsewhere. Master's and doctoral candidates should take care to notice if their library's discovery tool automatically limits to available materials and broaden the scope to include ALL materials, not just those available.

With the foregoing in mind, a researcher might start a search by using the library's discovery tool and then follow up by reviewing which databases have been included in the search and, more importantly, which databases have not been included. Most libraries will facilitate locating its individual databases through a subject arrangement of some kind. Once those databases that are not discoverable have been identified, the researcher would do well to search them individually to find out if other materials can be identified outside of the discovery tool. One additional tool that a doctoral researcher should of necessity include in a search is ISI's  Web of Knowledge . The two major systems searchable within ISI's  Web  are the  Social Sciences Citation Index  and the  Science Citation Index . The purpose of these two systems is to enable a researcher to determine what research has been cited over the years by any number of researchers and how many times it has been cited.

Formulating an Effective Search Strategy -- Key to performing an effective literature review is selecting search terms that will effectively identify materials that are relevant to the research topic. An initial strategy for selecting search terminology might be to list all possible relevant terms and their synonyms in order to have a working vocabulary for use in the research databases. While an individual subject database will likely use a "controlled vocabulary" to index articles and other materials that are included in the database, the same vocabulary might not be as effective in a database that focuses on a different subject area. For example, terminology that is used frequently in psychological literature might not be as effective in searching a human resources management database. Brainstorming the topic before launching into a search will help a researcher arrive at a good working vocabulary to use when probing the databases for relevant literature.

As materials are identified with the initial search, the researcher will want to keep track of other terminology that could be of use in performing additional searches. Sometimes the most effective search terminology can be found by reading the abstracts of relevant materials located through a library's research databases. For example, an initial search on the concept of "mainstreaming" might lead the researcher to articles that discuss mainstreaming but which also look into the concept of "inclusion" in education. While the terms mainstreaming and inclusion are sometimes used synonymously, they really embody two different approaches to working with students having special needs. Abstracts of articles located in the initial search on mainstreaming will uncover related concepts such as inclusion and help a researcher develop a better, more effective vocabulary for fleshing out the literature review.

In addition to searching using key concepts aligned with the research topic, a researcher likely also will want to search for additional materials produced by key authors who are identified in the initial searches. As a researcher reviews items retrieved in the initial stages of the survey, he or she will begin to notice certain authors coming up over and over in relation to the topic. To make sure that no stone is left unturned, it would be advisable to search the available, relevant library databases for other materials by those key authors, just to make sure something of importance has not been missed. A review of the reference lists for each of the items identified in the search will also help to identify key literature that should be reviewed.

Locating the Materials and Composing the Review -- In many cases the items identified through the library's databases will also be available online through the same or related databases. This, however, is not always the case. When materials are not available online, the researcher should check the library's physical collections (print, media, etc.) to determine if the items are available in the library, itself. For those materials not physically available in the home library, the researcher will use interlibrary loan to procure copies from other libraries or services. While abstracts are extremely useful in identifying the right types of materials, they are no substitute for the actual items, themselves. The thorough researcher will make sure that all the key literature has been retrieved and read thoroughly before proceeding too far with the original research.

The end result of the literature review is a discussion of the central themes in the research and an overview of the significant studies located by the researcher. This discussion serves as the lead section of a paper or article that reports the findings of an original research study and sets the stage for presentation of the original study by providing a review of research that has been conducted prior to the current study. As the researcher conducts his or her own study, other relevant materials might enter into the professional literature. It is the researcher's responsibility to update the literature review with newly released information prior to completing his or her own study.

Updating the Initial Search -- Most research projects will take place over a period of time and are not completed in the short term. Especially in the case of master's and doctoral projects, the research process might take a year or several years to complete. During this time, it will be important for the researcher to periodically review the research that has been going on at the same time as his or her own research. Revisiting the search strategies employed in the initial pass of the ltierature will turn up any new studies that might have come to light since the initial search. Fortunately, most research databases and discovery systems provide researchers with the means for automatically notifying them when new materials matching the search strategy have entered the system. This requires that a researcher sign up for a personal "account" with the database in order to save his or her searches and set up "alerts" when new materials come online. Setting up an account does not involve charges to the researcher; this is all a part of the cost borne by the home library in providing access to the databases.

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Five steps to conducting a systematic review

Regina kunz.

1 German Cochrane Centre, Freiburg and Department of Nephrology, Charité, Berlin, Germany

Jos Kleijnen

2 Centre for Reviews and Dissemination, York, UK

3 German Cochrane Centre, Freiburg, Germany

Systematic reviews and meta-analyses are a key element of evidence-based healthcare, yet they remain in some ways mysterious. Why did the authors select certain studies and reject others? What did they do to pool results? How did a bunch of insignificant findings suddenly become significant? This paper, along with a book 1 that goes into more detail, demystifies these and other related intrigues.

A review earns the adjective systematic if it is based on a clearly formulated question, identifies relevant studies, appraises their quality and summarizes the evidence by use of explicit methodology. It is the explicit and systematic approach that distinguishes systematic reviews from traditional reviews and commentaries. Whenever we use the term review in this paper it will mean a systematic review . Reviews should never be done in any other way.

In this paper we provide a step-by-step explanation—there are just five steps—of the methods behind reviewing, and the quality elements inherent in each step (Box 1). For purposes of illustration we use a published review concerning the safety of public water fluoridation, but we must emphasize that our subject is review methodology, not fluoridation.

EXAMPLE: SAFETY OF PUBLIC WATER FLUORIDATION

You are a public health professional in a locality that has public water fluoridation. For many years, your colleagues and you have believed that it improves dental health. Recently there has been pressure from various interest groups to consider the safety of this public health intervention because they fear that it is causing cancer. Public health decisions have been based on professional judgment and practical feasibility without explicit consideration of the scientific evidence. (This was yesterday; today the evidence is available in a York review 2 , 3 , identifiable on MEDLINE through the freely accessible PubMed clinical queries interface [ http://www.ncbi.nlm.nib.gov/entrez/query/static/clinical.html ], under ‘systematic reviews’.)

STEP 1: FRAMING THE QUESTION

The research question may initially be stated as a query in free form but reviewers prefer to pose it in a structured and explicit way. The relations between various components of the question and the structure of the research design are shown in Figure 1 . This paper focuses only on the question of safety related to the outcomes described below.

Structured questions for systematic reviews and relations between question components in a comparative study

Box 1 The steps in a systematic review

The problems to be addressed by the review should be specified in the form of clear, unambiguous and structured questions before beginning the review work. Once the review questions have been set, modifications to the protocol should be allowed only if alternative ways of defining the populations, interventions, outcomes or study designs become apparent

The search for studies should be extensive. Multiple resources (both computerized and printed) should be searched without language restrictions. The study selection criteria should flow directly from the review questions and be specified a priori . Reasons for inclusion and exclusion should be recorded

Study quality assessment is relevant to every step of a review. Question formulation (Step 1) and study selection criteria (Step 2) should describe the minimum acceptable level of design. Selected studies should be subjected to a more refined quality assessment by use of general critical appraisal guides and design-based quality checklists (Step 3). These detailed quality assessments will be used for exploring heterogeneity and informing decisions regarding suitability of meta-analysis (Step 4). In addition they help in assessing the strength of inferences and making recommendations for future research (Step 5)

Data synthesis consists of tabulation of study characteristics, quality and effects as well as use of statistical methods for exploring differences between studies and combining their effects (meta-analysis). Exploration of heterogeneity and its sources should be planned in advance (Step 3). If an overall meta-analysis cannot be done, subgroup meta-analysis may be feasible

The issues highlighted in each of the four steps above should be met. The risk of publication bias and related biases should be explored. Exploration for heterogeneity should help determine whether the overall summary can be trusted, and, if not, the effects observed in high-quality studies should be used for generating inferences. Any recommendations should be graded by reference to the strengths and weaknesses of the evidence

Free-form question

Is it safe to provide population-wide drinking water fluoridation to prevent caries?

Structured question

• The populations —Populations receiving drinking water sourced through a public water supply
• The interventions or exposures —Fluoridation of drinking water (natural or artificial) compared with non-fluoridated water
• The outcomes —Cancer is the main outcome of interest for the debate in your health authority
• The study designs —Comparative studies of any design examining the harmful outcomes in at least two population groups, one with fluoridated drinking water and the other without. Harmful outcomes can be rare and they may develop over a long time. There are considerable difficulties in designing and conducting safety studies to capture these outcomes, since a large number of people need to be observed over a long period. These circumstances demand observational, not randomized studies. With this background, systematic reviews on safety have to include evidence from studies with a range of designs.

STEP 2: IDENTIFYING RELEVANT PUBLICATIONS

To capture as many relevant citations as possible, a wide range of medical, environmental and scientific databases were searched to identify primary studies of the effects of water fluoridation. The electronic searches were supplemented by hand searching of Index Medicus and Excerpta Medica back to 1945. Furthermore, various internet engines were searched for web pages that might provide references. This effort resulted in 3246 citations from which relevant studies were selected for the review. Their potential relevance was examined, and 2511 citations were excluded as irrelevant. The full papers of the remaining 735 citations were assessed to select those primary studies in man that directly related to fluoride in drinking water supplies, comparing at least two groups. These criteria excluded 481 studies and left 254 in the review. They came from thirty countries, published in fourteen languages between 1939 and 2000. Of these studies 175 were relevant to the question of safety, of which 26 used cancer as an outcome.

STEP 3: ASSESSING STUDY QUALITY

Design threshold for study selection.

Adequate study design as a marker of quality, is listed as an inclusion criterion in Box 1. This approach is most applicable when the main source of evidence is randomized studies. However, randomized studies are almost impossible to conduct at community level for a public health intervention such as water fluoridation. Thus, systematic reviews assessing the safety of such interventions have to include evidence from a broader range of study designs. Consideration of the type and amount of research likely to be available led to inclusion of comparative studies of any design. In this way, selected studies provided information about the harmful effects of exposure to fluoridated water compared with non-exposure.

Quality assessment of safety studies

After studies of an acceptable design have been selected, their in-depth assessment for the risk of various biases allows us to gauge the quality of the evidence in a more refined way. Biases either exaggerate or underestimate the ‘true’ effect of an exposure. The objective of the included studies was to compare groups exposed to fluoridated drinking water and those without such exposure for rates of undesirable outcomes, without bias. Safety studies should ascertain exposures and outcomes in such a way that the risk of misclassification is minimized. The exposure is likely to be more accurately ascertained if the study was prospective rather than retrospective and if it was started soon after water fluoridation rather than later. The outcomes of those developing cancer (and remaining free of cancer) are likely to be more accurately ascertained if the follow-up was long and if the assessment was blind to exposure status.

When examining how the effect of exposure on outcome was established, reviewers assessed whether the comparison groups were similar in all respects other than their exposure to fluoridated water. This is because the other differences may be related to the outcomes of interest independent of the drinking-water fluoridation, and this would bias the comparison. For example, if the people exposed to fluoridated water had other risk factors that made them more prone to have cancer, the apparent association between exposure and outcome might be explained by the more frequent occurrence of these factors among the exposed group. The technical word for such defects is confounding. In a randomized study, confounding factors are expected to be roughly equally distributed between groups. In observational studies their distribution may be unequal. Primary researchers can statistically adjust for these differences, when estimating the effect of exposure on outcomes, by use of multivariable modelling.

Put simply, use of a prospective design, robust ascertainment of exposure and outcomes, and control for confounding are the generic issues one would look for in quality assessment of studies on safety. Consequently, studies may range from satisfactorily meeting quality criteria, to having some deficiencies, to not meeting the criteria at all, and they can be assigned to one of three prespecified quality categories as shown in Table 1 . A quality hierarchy can then be developed, based on the degree to which studies comply with the criteria. None of the studies on cancer were in the high-quality category, but this was because randomized studies were non-existent and control for confounding was not always ideal in the observational studies. There were 8 studies of moderate quality and 18 of low quality.

Description of quality assessment of studies on safety of public water fluoridation

STEP 4: SUMMARIZING THE EVIDENCE

To summarize the evidence from studies of variable design and quality is not easy. The original review 3 provides details of how the differences between study results were investigated and how they were summarized (with or without meta-analysis). This paper restricts itself to summarizing the findings narratively. The association between exposure to fluoridated water and cancer in general was examined in 26 studies. Of these, 10 examined all-cause cancer incidence or mortality, in 22 analyses. Of these, 11 analyses found a negative association (fewer cancers due to exposure), 9 found a positive one and 2 found no association. Only 2 studies reported statistically significant differences. Thus no clear association between water fluoridation and increased cancer incidence or mortality was apparent. Bone/joint and thyroid cancers were of particular concern because of fluoride uptake by these organs. Neither the 6 studies of osteosarcoma nor the 2 studies of thyroid cancer and water fluoridation revealed significant differences. Overall no association was detected between water fluoridation and mortality from any cancer. These findings were also borne out in the moderate-quality subgroup of studies.

STEP 5: INTERPRETING THE FINDINGS

In the fluoridation example, the focus was on the safety of a community-based public health intervention. The generally low quality of available studies means that the results must be interpreted with caution. However, the elaborate efforts in searching an unusually large number of databases provide some safeguard against missing relevant studies. Thus the evidence summarized in this review is likely to be as good as it will get in the foreseeable future. Cancer was the harmful outcome of most interest in this instance. No association was found between exposure to fluoridated water and specific cancers or all cancers. The interpretation of the results may be generally limited because of the low quality of studies, but the findings for the cancer outcomes are supported by the moderate-quality studies.

After having spent some time reading and understanding the review, you are impressed by the sheer amount of published work relevant to the question of safety. However, you are somewhat disappointed by the poor quality of the primary studies. Of course, examination of safety only makes sense in a context where the intervention has some beneficial effect. Benefit and harm have to be compared to provide the basis for decision making. On the issue of the beneficial effect of public water fluoridation, the review 3 reassures you that the health authority was correct in judging that fluoridation of drinking water prevents caries. From the review you also discovered that dental fluorosis (mottled teeth) was related to concentration of fluoride. When the interest groups raise the issue of safety again, you will be able to declare that there is no evidence to link cancer with drinking-water fluoridation; however, you will have to come clean about the risk of dental fluorosis, which appears to be dose dependent, and you may want to measure the fluoride concentration in the water supply and share this information with the interest groups.

The ability to quantify the safety concerns of your population through a review, albeit from studies of moderate to low quality, allows your health authority, the politicians and the public to consider the balance between beneficial and harmful effects of water fluoridation. Those who see the prevention of caries as of primary importance will favour fluoridation. Others, worried about the disfigurement of mottled teeth, may prefer other means of fluoride administration or even occasional treatment for dental caries. Whatever the opinions on this matter, you are able to reassure all parties that there is no evidence that fluoridation of drinking water increases the risk of cancer.

With increasing focus on generating guidance and recommendations for practice through systematic reviews, healthcare professionals need to understand the principles of preparing such reviews. Here we have provided a brief step-by-step explanation of the principles. Our book 1 describes them in detail.