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How Long Should a Literature Review Be?

4-minute read

• 7th October 2023

If you’re writing a research paper or dissertation , then you know how important it is to include a thorough, comprehensive literature review. But exactly how long should your literature review be in relation to the rest of your work? While there’s no one-size-fits-all answer to that question, there are some factors that will help determine the length of your review. In this post, we’ll discuss what information to include in your literature review and how long it should be.

Keep reading to learn more.

What Is a Literature Review?

A literature review is a critical summary and evaluation of the current resources (e.g., books and journal articles) on a specific topic or research question. It is a crucial part of academic writing, such as dissertations, in all categories and fields. Essentially, literature reviews help contextualize your investigations and show how your work is building on existing research.

No matter how long your literature review is, it should generally:

●  Establish context for your research (i.e., provide relevant background information so your reader understands the historical significance of your study ).

●  Identify gaps in the existing literature (such as unaddressed questions or aspects of your topic).

●  Highlight significant concepts related to your topic.

●  Cite relevant studies.

●  Support your argument.

It’s also essential that a literature review critically analyze the sources cited in your study, considering factors such as sample size, research design, and potential biases. Be sure to structure your literature review using the same referencing style as the rest of your research paper (e.g., APA , Chicago , MLA ).

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The length of your literature review depends on several factors, including the scope and purpose of your research. In general, the length of the review should be proportionate to your overall paper. For example, if you’re writing a fifty-thousand-word dissertation, then your literature review will likely be an entire chapter comprising about 20 pages. If it’s for a 15-page research paper, your literature review may only be a few pages.

Here are several factors that could affect the length of your literature review:

●  Institutional guidelines : Always check the guidelines provided by your institution or journal (such as an APA journal ). There may be a specific length or word count required for publication.

●  Scope : If your research topic is narrow and focused, your literature review may be shorter. Conversely, if your topic is broad and encompasses a large body of literature, your review may need to be longer.

●  Field of study : Different academic fields may have different expectations regarding the length of literature reviews. For example, literature reviews in the humanities might be longer than those in the natural sciences.

Also, consider your audience. If your literature review is for a general audience or a class assignment, it can probably be shorter and less specialized. However, if it’s for an academic audience in your field of study, you may need to be more thorough and provide an extensive review of the existing literature.

Most literature reviews follow the same basic structure of an introduction, body, and conclusion. Most of the time, they are part of a larger work, so the introduction and conclusion paragraphs will be relatively brief.

However, if the review is a standalone piece, then your introduction and conclusion will be longer since you will need to discuss your research objectives, methods, and findings as well as analyze the literature used in your study.

To ensure your literature review makes an impression, have it professionally proofread by our expert literature review editing services . Submit your free sample of 500 words or less to get started today!

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The Guide to Literature Reviews

• What is a Literature Review?
• The Purpose of Literature Reviews
• Guidelines for Writing a Literature Review
• How to Organize a Literature Review?
• Software for Literature Reviews
• Using Artificial Intelligence for Literature Reviews
• How to Conduct a Literature Review?
• Common Mistakes and Pitfalls in a Literature Review
• Methods for Literature Reviews
• What is a Systematic Literature Review?
• What is a Narrative Literature Review?
• What is a Descriptive Literature Review?
• What is a Scoping Literature Review?
• What is a Realist Literature Review?
• What is a Critical Literature Review?
• Meta Analysis vs. Literature Review
• What is an Umbrella Literature Review?
• Differences Between Annotated Bibliographies and Literature Reviews
• Literature Review vs. Theoretical Framework
• How to Write a Literature Review?
• How to Structure a Literature Review?
• How to Make a Cover Page for a Literature Review?
• How to Write an Abstract for a Literature Review?
• How to Write a Literature Review Introduction?
• How to Write the Body of a Literature Review?
• How to Write a Literature Review Conclusion?
• How to Make a Literature Review Bibliography?
• How to Format a Literature Review?

Introduction

What determines the length of a literature review, structure and organization, lengths in different kinds of literature reviews.

• Examples of Literature Reviews
• How to Present a Literature Review?
• How to Publish a Literature Review?

How Long Should a Literature Review Be?

A literature review is an essential part of many academic papers and research projects. It provides a comprehensive overview of existing research on a particular topic, allowing the researcher to identify gaps, build on previous work, and position their study within the broader academic context. The length of a literature review can vary significantly depending on the context, purpose, and research scope . In this article, we will explore the factors that influence the length of a literature review and provide guidelines to help you determine the appropriate length for your work.

The length of a literature review is influenced by multiple factors, including the type of review, the research aims, the scope of the topic, and institutional guidelines. Writing a literature review involves synthesizing existing research and providing a critical analysis, which varies depending on the depth and breadth of the subject. Additionally, a literature review involves evaluating and summarizing scholarly sources to support the research objectives. Understanding these factors is crucial for researchers to effectively plan and structure their reviews, ensuring they meet academic standards and adequately cover the relevant literature.

Context and purpose

In most literature reviews, the length largely depends on the context and purpose of your research. Here are some common scenarios to consider:

Undergraduate research papers : Typically concise, these reviews range from 1,000 to 3,000 words (4-12 pages) and provide a basic overview of the existing literature. They are usually integrated into the introduction or background section of the paper.

Master's theses : More detailed and comprehensive, literature reviews for master's theses usually span 3,000 to 10,000 words (10-40 pages). They demonstrate the student's understanding of the field and justify the research question.

Doctoral dissertations : The literature review can be extensive for postgraduate dissertations, often ranging from 8,000 to 20,000 words (30-80 pages). These reviews need to cover a wide range of studies and theories, providing a thorough background for the research and highlighting gaps the dissertation aims to address.

Journal articles : Concise and focused, literature reviews in journal articles typically range from 2,000 to 5,000 words (8-20 pages), depending on the journal's guidelines. They concentrate on recent and relevant studies that directly inform the research question.

Grant proposals : Brief and focused, literature reviews in grant proposals are usually around 1,000 to 2,000 words (4-8 pages). Their purpose is to provide a quick overview of the current state of research and justify the need for the proposed study.

Research scope

The breadth and depth of the literature you need to cover will also affect the length of your review. If your research focuses on a specific niche or well-defined topic, your literature review might be shorter, around 1,000 to 3,000 words. This is because there may be fewer studies to review, allowing you to focus on the most relevant ones. For topics that span multiple disciplines or have a vast body of literature, the review will be longer, potentially exceeding 10,000 words. A broader scope requires a more extensive review to ensure all relevant research is covered.

Guidelines and requirements

Always check the guidelines provided by your institution, advisor, or the journal to which you are submitting. These guidelines often specify the expected length and format for the literature review. Adhering to these requirements is crucial for ensuring your review meets academic standards and is accepted for publication or evaluation.

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A well-structured literature review typically includes the following sections, which can influence its length:

Provides an overview of the topic and the review's purpose. This section usually accounts for 5-10% of the total word count. In a 3,000-word review, the introduction might be around 150-300 words.

Discusses key themes, theories, and findings in the literature. This section is the core of the review and can be divided into sub-sections based on themes, methodologies, or chronological order. It usually makes up 70-80% of the total word count. For example, in a 3,000-word review, the main body might be around 2,100-2,400 words.

Summarizes the key findings and highlights gaps or areas for future research. This section usually accounts for 10-15% of the total word count. In a 3,000-word review, the conclusion might be around 300-450 words.

Different methods of conducting literature reviews affect the length. The length of a literature review depends on several factors, including the chosen approach, institutional guidelines, and the specific requirements of the research project. Systematic reviews are usually the most extensive due to their rigorous and comprehensive nature, while narrative reviews are more flexible and shorter. Scoping reviews provide a broad overview without the detailed synthesis required by systematic reviews, placing them in the middle range of length. Umbrella reviews and realist reviews vary in length but generally provide a comprehensive synthesis of existing reviews or focus on the mechanisms of interventions.

Understanding these differences helps researchers select the appropriate approach and determine the optimal length for their literature review, ensuring it meets academic standards and contributes meaningfully to the field of study.

The length of a literature review is influenced by several factors, including the type of review, the scope of the research topic, and institutional guidelines. Most literature reviews vary in length depending on the specific requirements of the research project. The process of literature review writing is crucial for determining length, as it involves synthesizing research articles, current research, and existing scholarship to adequately cover the topic. For instance, a thesis literature review tends to be more extensive compared to a literature review for a research paper or journal article.

When determining how long a literature review should be, it's essential to consider the need to address current theories and provide a critical analysis of scholarly sources. Systematic reviews are typically more detailed and extensive, often requiring a substantial number of pages, while narrative reviews may only span a few pages. The length is also shaped by the research aims and the depth of coverage needed for the original research. Creating an annotated bibliography also affects the length of the review. Adhering to institutional guidelines is crucial, as they often provide specific directions on the expected length and structure of the review.

Ultimately, the length of a literature review is dictated by the comprehensive coverage required to support the research aims and the specific nature of the research project. Whether it's a thesis literature review, a journal article, or part of a larger research paper, the goal is to ensure the review is thorough, insightful, and aligned with the objectives of the current project. By understanding these factors, researchers can determine the optimal length for their literature review, ensuring it meets academic standards and effectively contributes to the field of study.

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Reference management. Clean and simple.

How to write a literature review in 6 steps

What is a literature review?

How to write a literature review, 1. determine the purpose of your literature review, 2. do an extensive search, 3. evaluate and select literature, 4. analyze the literature, 5. plan the structure of your literature review, 6. write your literature review, other resources to help you write a successful literature review, frequently asked questions about writing a literature review, related articles.

A literature review is an assessment of the sources in a chosen topic of research.

A good literature review does not just summarize sources. It analyzes the state of the field on a given topic and creates a scholarly foundation for you to make your own intervention. It demonstrates to your readers how your research fits within a larger field of study.

In a thesis, a literature review is part of the introduction, but it can also be a separate section. In research papers, a literature review may have its own section or it may be integrated into the introduction, depending on the field.

➡️ Our guide on what is a literature review covers additional basics about literature reviews.

• Identify the main purpose of the literature review.
• Do extensive research.
• Evaluate and select relevant sources.
• Analyze the sources.
• Plan a structure.
• Write the review.

In this section, we review each step of the process of creating a literature review.

In the first step, make sure you know specifically what the assignment is and what form your literature review should take. Read your assignment carefully and seek clarification from your professor or instructor if needed. You should be able to answer the following questions:

• How many sources do I need to include?
• What types of sources should I review?
• Should I evaluate the sources?
• Should I summarize, synthesize or critique sources?
• Do I need to provide any definitions or background information?

In addition to that, be aware that the narrower your topic, the easier it will be to limit the number of sources you need to read in order to get a good overview of the topic.

Now you need to find out what has been written on the topic and search for literature related to your research topic. Make sure to select appropriate source material, which means using academic or scholarly sources , including books, reports, journal articles , government documents and web resources.

➡️ If you’re unsure about how to tell if a source is scholarly, take a look at our guide on how to identify a scholarly source .

Come up with a list of relevant keywords and then start your search with your institution's library catalog, and extend it to other useful databases and academic search engines like:

• Google Scholar
• Science.gov

➡️ Our guide on how to collect data for your thesis might be helpful at this stage of your research as well as the top list of academic search engines .

Once you find a useful article, check out the reference list. It should provide you with even more relevant sources. Also, keep a note of the:

• authors' names
• page numbers

Keeping track of the bibliographic information for each source will save you time when you’re ready to create citations. You could also use a reference manager like Paperpile to automatically save, manage, and cite your references.

Read the literature. You will most likely not be able to read absolutely everything that is out there on the topic. Therefore, read the abstract first to determine whether the rest of the source is worth your time. If the source is relevant for your topic:

• Read it critically.
• Look for the main arguments.
• Take notes as you read.
• Organize your notes using a table, mind map, or other technique.

Now you are ready to analyze the literature you have gathered. While your are working on your analysis, you should ask the following questions:

• What are the key terms, concepts and problems addressed by the author?
• How is this source relevant for my specific topic?
• How is the article structured? What are the major trends and findings?
• What are the conclusions of the study?
• How are the results presented? Is the source credible?
• When comparing different sources, how do they relate to each other? What are the similarities, what are the differences?
• Does the study help me understand the topic better?
• Are there any gaps in the research that need to be filled? How can I further my research as a result of the review?

Tip: Decide on the structure of your literature review before you start writing.

There are various ways to organize your literature review:

• Chronological method : Writing in the chronological method means you are presenting the materials according to when they were published. Follow this approach only if a clear path of research can be identified.
• Thematic review : A thematic review of literature is organized around a topic or issue, rather than the progression of time.
• Publication-based : You can order your sources by publication, if the way you present the order of your sources demonstrates a more important trend. This is the case when a progression revealed from study to study and the practices of researchers have changed and adapted due to the new revelations.
• Methodological approach : A methodological approach focuses on the methods used by the researcher. If you have used sources from different disciplines that use a variety of research methods, you might want to compare the results in light of the different methods and discuss how the topic has been approached from different sides.

Regardless of the structure you chose, a review should always include the following three sections:

• An introduction, which should give the reader an outline of why you are writing the review and explain the relevance of the topic.
• A body, which divides your literature review into different sections. Write in well-structured paragraphs, use transitions and topic sentences and critically analyze each source for how it contributes to the themes you are researching.
• A conclusion , which summarizes the key findings, the main agreements and disagreements in the literature, your overall perspective, and any gaps or areas for further research.

➡️ If your literature review is part of a longer paper, visit our guide on what is a research paper for additional tips.

➡️ UNC writing center: Literature reviews

➡️ How to write a literature review in 3 steps

➡️ How to write a literature review in 30 minutes or less

The goal of a literature review is to asses the state of the field on a given topic in preparation for making an intervention.

A literature review should have its own independent section. You should indicate clearly in the table of contents where it can be found, and address this section as “Literature Review.”

There is no set amount of words for a literature review; the length depends on the research. If you are working with a large amount of sources, then it will be long. If your paper does not depend entirely on references, then it will be short.

Most research papers include a literature review. By assessing the available sources in your field of research, you will be able to make a more confident argument about the topic.

Literature reviews are most commonly found in theses and dissertations. However, you find them in research papers as well.

How long should a literature review be? Writing a literature review properly. 

Embarking on a journey through academic research, one is often faced with the question: How long should a thesis literature review be?

Typically, a literature review comprises 20-40% of the thesis, equating to around 20-40 pages, yet this can fluctuate based on the topic, field, and institutional guidelines. However, if the literature review is for an assignment/project report it only needs to be a few pages long.

A literature review represents a critical exploration of existing scholarship on a specific topic, serving as the backbone for dissertations, research articles, book chapters, conference papers, and more.

This comprehensive, concise, and well-structured survey of the current knowledge landscape provides insight into established findings, research methodologies, and highlights the gaps that your research aims to fill.

The endeavor of writing a literature review not only deepens your understanding of your chosen area but also lays a strong foundation for your unique inquiry and contribution to the field.

How Long Should a Thesis Literature Review Be? Dissertation, research paper, journal article, and more

The length of a thesis literature review can vary based on the topic, research field, and guidelines provided by your institution.

However, as a general rule, it often comprises 20-40% of the thesis. This equates to around 20-40 pages.

However, if the literature review is for an assignment it only needs to be a few pages long. 

It’s crucial to ensure your review is comprehensive, concise, and well-structured, adequately summarizing existing research, identifying gaps, discussing limitations, and suggesting future research directions. 

What is a literature review?

A literature review is a critical examination of existing research on a particular topic. It is often the beginning of any area of study and is often used as the introduction to a dissertation. 

I have done one at the beginning of any research project, the beginning of my PhD and postdocs and in industry research positions. 

It involves analyzing relevant resources such as books, journal articles, and other scholarly works to gain a comprehensive understanding of the current knowledge in your area of interest.

For example, if your topic is the impact of social media on the mental health of younger generations, you’ll explore previous studies to understand established findings, methodologies, and gaps in the field.

The purpose is threefold: 

• to comprehend what is already known,
• to give your readers an overview of existing knowledge (showing where your research fits in), and
• to identify any unexplored areas or gaps, thus allowing you to contribute something original to the field.

The process aids in refining your research question and defining your unique angle of inquiry.

How do I create a literature review?

Creating a literature review involves several key steps:

• Conceptualize : Start by forming a conceptual framework. It’s an overview of the topic, helping you structure your review.
• Synthesis : Synthesize the literature. Read, categorize, and summarize the material to create a narrative.
• Analysis : This is your main body of work. Develop arguments and critique the literature.
• Conclusion : Recap your findings, discussing the implications of your work and highlighting any limitations.
• Future Research : If applicable, suggest areas for future research or policy actions based on your findings.
• Introduction : Write the introduction last. It includes the importance of the topic, gaps in knowledge, and your motivation for the review.
• Transparency : Always be transparent about limitations in your work and the body of evidence reviewed.

How many sources do you need in a literature review?

The number of sources required in a literature review can vary significantly depending on several factors such as:

• the nature of your research topic,
• the length of the literature review, and
• the specific instructions from your professor or institution.

In general, it’s crucial to include a wide range of sources to fully capture the breadth of scholarship on your research topic.

For a dissertation or thesis, a literature review might involve dozens of sources. For a stand-alone literature review or an overview for a journal article, fewer sources might be appropriate. For instance, a PhD thesis literature review might require anywhere from 50 to 300+ sources.

My PhD thesis had 256 papers cited

Rather than focusing on a specific number, the key is to ensure that your literature review provides a comprehensive, balanced overview of existing scholarship on your research topic.

This should ideally include both seminal works and recent publications, highlighting any gaps your research aims to fill.

Remember, the quality of your sources is as important as the quantity.

Your literature review should demonstrate that you have critically engaged with a variety of scholarly sources to gain a deep understanding of your research topic.

What Makes a Good Literature Review?

A good literature review for a dissertation or thesis is a comprehensive survey of existing scholarship on a particular topic, demonstrating your understanding of current research in your field of study.

The length of a literature review can vary depending on the research topic and level of study, but the University of Kent suggests that for a PhD thesis, it could range up to 20 pages or more.

Start by organizing your literature review either thematically, chronologically, or by methodology.

An annotated bibliography can assist in this process.

You must summarize the main points of each source, focusing on its relevance to your research project. Scholarly sources are preferable; consult your instructor for the minimum number of sources required.

It is essential to identify a gap in knowledge within the existing scholarship, which your research aims to fill.

This should be highlighted in your review, underlining the significance and worthiness of your proposed research. It’s crucial that your review provides a complete overview of the current state of research and shows why your research is important.

Bear in mind the word count, as a concise, focused review is typically preferred.

Structure is key and the title page, introduction, and conclusion should be included. 

Despite the absence of a hard and fast rule on length, your review must provide a robust base of knowledge for your research, allowing readers to understand your research’s purpose and audience.

Wrapping up – How long should a literature review be?

The question “how long should a literature review be?” hinges on various factors, including the nature of the research project, area of study, and instructions from your instructor.

Whether it’s for a thesis, dissertation, or a research paper, writing a literature review requires a comprehensive survey of existing scholarship on your particular topic.

Remember, the length of a literature review is not a hard and fast rule; it could comprise anywhere from 20 pages to much more in a PhD thesis, or be a stand-alone brief overview in a journal article.

The structure of your review, whether thematically, chronologically, or by methodology, helps to summarize and highlight specific findings in the field of study.

Bear in mind the importance of identifying a gap in knowledge, which your research aims to fill.

Above all, your literature review should be a concise, scholarly synthesis of your research topic, providing a robust base of knowledge for your audience and demonstrating the significance and worthiness of your proposed research. 

Dr Andrew Stapleton has a Masters and PhD in Chemistry from the UK and Australia. He has many years of research experience and has worked as a Postdoctoral Fellow and Associate at a number of Universities. Although having secured funding for his own research, he left academia to help others with his YouTube channel all about the inner workings of academia and how to make it work for you.

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Open Access

Ten Simple Rules for Writing a Literature Review

* E-mail: [email protected]

Affiliations Centre for Functional and Evolutionary Ecology (CEFE), CNRS, Montpellier, France, Centre for Biodiversity Synthesis and Analysis (CESAB), FRB, Aix-en-Provence, France

• Marco Pautasso

Published: July 18, 2013

• https://doi.org/10.1371/journal.pcbi.1003149
• Reader Comments

Citation: Pautasso M (2013) Ten Simple Rules for Writing a Literature Review. PLoS Comput Biol 9(7): e1003149. https://doi.org/10.1371/journal.pcbi.1003149

Editor: Philip E. Bourne, University of California San Diego, United States of America

Copyright: © 2013 Marco Pautasso. This is an open-access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.

Funding: This work was funded by the French Foundation for Research on Biodiversity (FRB) through its Centre for Synthesis and Analysis of Biodiversity data (CESAB), as part of the NETSEED research project. The funders had no role in the preparation of the manuscript.

Competing interests: The author has declared that no competing interests exist.

Literature reviews are in great demand in most scientific fields. Their need stems from the ever-increasing output of scientific publications [1] . For example, compared to 1991, in 2008 three, eight, and forty times more papers were indexed in Web of Science on malaria, obesity, and biodiversity, respectively [2] . Given such mountains of papers, scientists cannot be expected to examine in detail every single new paper relevant to their interests [3] . Thus, it is both advantageous and necessary to rely on regular summaries of the recent literature. Although recognition for scientists mainly comes from primary research, timely literature reviews can lead to new synthetic insights and are often widely read [4] . For such summaries to be useful, however, they need to be compiled in a professional way [5] .

When starting from scratch, reviewing the literature can require a titanic amount of work. That is why researchers who have spent their career working on a certain research issue are in a perfect position to review that literature. Some graduate schools are now offering courses in reviewing the literature, given that most research students start their project by producing an overview of what has already been done on their research issue [6] . However, it is likely that most scientists have not thought in detail about how to approach and carry out a literature review.

Reviewing the literature requires the ability to juggle multiple tasks, from finding and evaluating relevant material to synthesising information from various sources, from critical thinking to paraphrasing, evaluating, and citation skills [7] . In this contribution, I share ten simple rules I learned working on about 25 literature reviews as a PhD and postdoctoral student. Ideas and insights also come from discussions with coauthors and colleagues, as well as feedback from reviewers and editors.

Rule 1: Define a Topic and Audience

How to choose which topic to review? There are so many issues in contemporary science that you could spend a lifetime of attending conferences and reading the literature just pondering what to review. On the one hand, if you take several years to choose, several other people may have had the same idea in the meantime. On the other hand, only a well-considered topic is likely to lead to a brilliant literature review [8] . The topic must at least be:

• interesting to you (ideally, you should have come across a series of recent papers related to your line of work that call for a critical summary),
• an important aspect of the field (so that many readers will be interested in the review and there will be enough material to write it), and
• a well-defined issue (otherwise you could potentially include thousands of publications, which would make the review unhelpful).

Ideas for potential reviews may come from papers providing lists of key research questions to be answered [9] , but also from serendipitous moments during desultory reading and discussions. In addition to choosing your topic, you should also select a target audience. In many cases, the topic (e.g., web services in computational biology) will automatically define an audience (e.g., computational biologists), but that same topic may also be of interest to neighbouring fields (e.g., computer science, biology, etc.).

Rule 2: Search and Re-search the Literature

After having chosen your topic and audience, start by checking the literature and downloading relevant papers. Five pieces of advice here:

• keep track of the search items you use (so that your search can be replicated [10] ),
• keep a list of papers whose pdfs you cannot access immediately (so as to retrieve them later with alternative strategies),
• use a paper management system (e.g., Mendeley, Papers, Qiqqa, Sente),
• define early in the process some criteria for exclusion of irrelevant papers (these criteria can then be described in the review to help define its scope), and
• do not just look for research papers in the area you wish to review, but also seek previous reviews.

The chances are high that someone will already have published a literature review ( Figure 1 ), if not exactly on the issue you are planning to tackle, at least on a related topic. If there are already a few or several reviews of the literature on your issue, my advice is not to give up, but to carry on with your own literature review,

• PPT PowerPoint slide
• PNG larger image
• TIFF original image

The bottom-right situation (many literature reviews but few research papers) is not just a theoretical situation; it applies, for example, to the study of the impacts of climate change on plant diseases, where there appear to be more literature reviews than research studies [33] .

https://doi.org/10.1371/journal.pcbi.1003149.g001

• discussing in your review the approaches, limitations, and conclusions of past reviews,
• trying to find a new angle that has not been covered adequately in the previous reviews, and
• incorporating new material that has inevitably accumulated since their appearance.

When searching the literature for pertinent papers and reviews, the usual rules apply:

• be thorough,
• use different keywords and database sources (e.g., DBLP, Google Scholar, ISI Proceedings, JSTOR Search, Medline, Scopus, Web of Science), and
• look at who has cited past relevant papers and book chapters.

Rule 3: Take Notes While Reading

If you read the papers first, and only afterwards start writing the review, you will need a very good memory to remember who wrote what, and what your impressions and associations were while reading each single paper. My advice is, while reading, to start writing down interesting pieces of information, insights about how to organize the review, and thoughts on what to write. This way, by the time you have read the literature you selected, you will already have a rough draft of the review.

Of course, this draft will still need much rewriting, restructuring, and rethinking to obtain a text with a coherent argument [11] , but you will have avoided the danger posed by staring at a blank document. Be careful when taking notes to use quotation marks if you are provisionally copying verbatim from the literature. It is advisable then to reformulate such quotes with your own words in the final draft. It is important to be careful in noting the references already at this stage, so as to avoid misattributions. Using referencing software from the very beginning of your endeavour will save you time.

Rule 4: Choose the Type of Review You Wish to Write

After having taken notes while reading the literature, you will have a rough idea of the amount of material available for the review. This is probably a good time to decide whether to go for a mini- or a full review. Some journals are now favouring the publication of rather short reviews focusing on the last few years, with a limit on the number of words and citations. A mini-review is not necessarily a minor review: it may well attract more attention from busy readers, although it will inevitably simplify some issues and leave out some relevant material due to space limitations. A full review will have the advantage of more freedom to cover in detail the complexities of a particular scientific development, but may then be left in the pile of the very important papers “to be read” by readers with little time to spare for major monographs.

There is probably a continuum between mini- and full reviews. The same point applies to the dichotomy of descriptive vs. integrative reviews. While descriptive reviews focus on the methodology, findings, and interpretation of each reviewed study, integrative reviews attempt to find common ideas and concepts from the reviewed material [12] . A similar distinction exists between narrative and systematic reviews: while narrative reviews are qualitative, systematic reviews attempt to test a hypothesis based on the published evidence, which is gathered using a predefined protocol to reduce bias [13] , [14] . When systematic reviews analyse quantitative results in a quantitative way, they become meta-analyses. The choice between different review types will have to be made on a case-by-case basis, depending not just on the nature of the material found and the preferences of the target journal(s), but also on the time available to write the review and the number of coauthors [15] .

Rule 5: Keep the Review Focused, but Make It of Broad Interest

Whether your plan is to write a mini- or a full review, it is good advice to keep it focused 16 , 17 . Including material just for the sake of it can easily lead to reviews that are trying to do too many things at once. The need to keep a review focused can be problematic for interdisciplinary reviews, where the aim is to bridge the gap between fields [18] . If you are writing a review on, for example, how epidemiological approaches are used in modelling the spread of ideas, you may be inclined to include material from both parent fields, epidemiology and the study of cultural diffusion. This may be necessary to some extent, but in this case a focused review would only deal in detail with those studies at the interface between epidemiology and the spread of ideas.

While focus is an important feature of a successful review, this requirement has to be balanced with the need to make the review relevant to a broad audience. This square may be circled by discussing the wider implications of the reviewed topic for other disciplines.

Rule 6: Be Critical and Consistent

Reviewing the literature is not stamp collecting. A good review does not just summarize the literature, but discusses it critically, identifies methodological problems, and points out research gaps [19] . After having read a review of the literature, a reader should have a rough idea of:

• the major achievements in the reviewed field,
• the main areas of debate, and
• the outstanding research questions.

It is challenging to achieve a successful review on all these fronts. A solution can be to involve a set of complementary coauthors: some people are excellent at mapping what has been achieved, some others are very good at identifying dark clouds on the horizon, and some have instead a knack at predicting where solutions are going to come from. If your journal club has exactly this sort of team, then you should definitely write a review of the literature! In addition to critical thinking, a literature review needs consistency, for example in the choice of passive vs. active voice and present vs. past tense.

Rule 7: Find a Logical Structure

Like a well-baked cake, a good review has a number of telling features: it is worth the reader's time, timely, systematic, well written, focused, and critical. It also needs a good structure. With reviews, the usual subdivision of research papers into introduction, methods, results, and discussion does not work or is rarely used. However, a general introduction of the context and, toward the end, a recapitulation of the main points covered and take-home messages make sense also in the case of reviews. For systematic reviews, there is a trend towards including information about how the literature was searched (database, keywords, time limits) [20] .

How can you organize the flow of the main body of the review so that the reader will be drawn into and guided through it? It is generally helpful to draw a conceptual scheme of the review, e.g., with mind-mapping techniques. Such diagrams can help recognize a logical way to order and link the various sections of a review [21] . This is the case not just at the writing stage, but also for readers if the diagram is included in the review as a figure. A careful selection of diagrams and figures relevant to the reviewed topic can be very helpful to structure the text too [22] .

Rule 8: Make Use of Feedback

Reviews of the literature are normally peer-reviewed in the same way as research papers, and rightly so [23] . As a rule, incorporating feedback from reviewers greatly helps improve a review draft. Having read the review with a fresh mind, reviewers may spot inaccuracies, inconsistencies, and ambiguities that had not been noticed by the writers due to rereading the typescript too many times. It is however advisable to reread the draft one more time before submission, as a last-minute correction of typos, leaps, and muddled sentences may enable the reviewers to focus on providing advice on the content rather than the form.

Feedback is vital to writing a good review, and should be sought from a variety of colleagues, so as to obtain a diversity of views on the draft. This may lead in some cases to conflicting views on the merits of the paper, and on how to improve it, but such a situation is better than the absence of feedback. A diversity of feedback perspectives on a literature review can help identify where the consensus view stands in the landscape of the current scientific understanding of an issue [24] .

Rule 9: Include Your Own Relevant Research, but Be Objective

In many cases, reviewers of the literature will have published studies relevant to the review they are writing. This could create a conflict of interest: how can reviewers report objectively on their own work [25] ? Some scientists may be overly enthusiastic about what they have published, and thus risk giving too much importance to their own findings in the review. However, bias could also occur in the other direction: some scientists may be unduly dismissive of their own achievements, so that they will tend to downplay their contribution (if any) to a field when reviewing it.

In general, a review of the literature should neither be a public relations brochure nor an exercise in competitive self-denial. If a reviewer is up to the job of producing a well-organized and methodical review, which flows well and provides a service to the readership, then it should be possible to be objective in reviewing one's own relevant findings. In reviews written by multiple authors, this may be achieved by assigning the review of the results of a coauthor to different coauthors.

Rule 10: Be Up-to-Date, but Do Not Forget Older Studies

Given the progressive acceleration in the publication of scientific papers, today's reviews of the literature need awareness not just of the overall direction and achievements of a field of inquiry, but also of the latest studies, so as not to become out-of-date before they have been published. Ideally, a literature review should not identify as a major research gap an issue that has just been addressed in a series of papers in press (the same applies, of course, to older, overlooked studies (“sleeping beauties” [26] )). This implies that literature reviewers would do well to keep an eye on electronic lists of papers in press, given that it can take months before these appear in scientific databases. Some reviews declare that they have scanned the literature up to a certain point in time, but given that peer review can be a rather lengthy process, a full search for newly appeared literature at the revision stage may be worthwhile. Assessing the contribution of papers that have just appeared is particularly challenging, because there is little perspective with which to gauge their significance and impact on further research and society.

Inevitably, new papers on the reviewed topic (including independently written literature reviews) will appear from all quarters after the review has been published, so that there may soon be the need for an updated review. But this is the nature of science [27] – [32] . I wish everybody good luck with writing a review of the literature.

Acknowledgments

Many thanks to M. Barbosa, K. Dehnen-Schmutz, T. Döring, D. Fontaneto, M. Garbelotto, O. Holdenrieder, M. Jeger, D. Lonsdale, A. MacLeod, P. Mills, M. Moslonka-Lefebvre, G. Stancanelli, P. Weisberg, and X. Xu for insights and discussions, and to P. Bourne, T. Matoni, and D. Smith for helpful comments on a previous draft.

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