Manuel B. Garcia

Manuel B. Garcia serves as the Senior Director for Educational Technology and Digital Learning at FEU Institute of Technology, Manila, Philippines. Read More

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Part 2 · Guide 89 of 247

What Is Grey Literature, and When Should You Search for It?

Grey literature includes research and other evidence produced outside conventional academic and commercial publishing. Learn what counts as grey literature and when searching for it can materially improve a literature review.

By Manuel B. Garcia
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What Is Grey Literature? Guide 89 of 247
01 · The Question

What exactly counts as grey literature?

You search journal databases, find dozens of relevant articles, and begin to feel that you have mapped the literature. But some potentially important evidence may never have appeared in a conventional journal. A government agency may have published a technical report. A doctoral student may have completed a dissertation. Researchers may have presented preliminary findings at a conference. A trial may be registered but not yet published as a full article.

Materials such as these are commonly discussed under the broad label grey literature. The term can be confusing because it does not describe one research design, level of quality, or publication status. Instead, it generally refers to material produced outside conventional academic or commercial publishing channels.

The practical question is therefore not simply whether grey literature exists on your topic. It is whether searching for it could provide evidence that matters to the question you are trying to answer.

02 · The Short Answer

Grey literature is evidence outside conventional publishing channels

In Brief

Grey literature broadly refers to research, information, and other documents produced outside traditional commercial or academic publishing, including materials such as theses, dissertations, government and institutional reports, working papers, conference materials, trial registrations, and some unpublished or ongoing research.

You should search for it when those sources could contain evidence relevant to your question that ordinary bibliographic databases may miss, particularly when conducting comprehensive evidence syntheses, studying policy or practice, examining emerging topics, or trying to identify unpublished evidence. Not every literature review requires an exhaustive grey literature search.

03 · What You Need to Know

Understanding what grey literature includes and why it matters

Grey literature is defined more by how it is distributed than by what it studies

There is no single universally applied boundary around grey literature. Definitions vary across disciplines and methodological traditions. JBI describes it broadly as material outside the traditional, usually peer-reviewed academic publishing market, while the Cochrane Handbook uses the term for reports published outside traditional commercial publishing.

This means that "grey literature" is not a research method. It is not a synonym for qualitative research, non-peer-reviewed research, or low-quality research. The category primarily concerns how material is produced, disseminated, and discovered.

Peer-reviewed literature Research disseminated through conventional scholarly publishing channels, typically after formal editorial and peer-review processes.
Grey literature Research or other relevant information produced outside conventional academic or commercial publishing channels, often through institutions, governments, organizations, repositories, conferences, or research projects themselves.

The boundary can sometimes be fuzzy. Conference proceedings, for example, may undergo some form of review. A dissertation is an academic document and normally undergoes institutional examination, yet it is commonly classified as grey literature because it is not disseminated through the conventional journal publishing system. Classification alone therefore tells you little about methodological quality.

What kinds of documents can be grey literature?

Depending on the field and the purpose of the review, grey literature may include:

  • master's theses and doctoral dissertations;
  • government reports and policy documents;
  • reports from universities, research institutes, professional associations, charities, and nongovernmental organizations;
  • technical reports and research reports;
  • working papers;
  • conference abstracts, posters, papers, and proceedings;
  • clinical trial registrations and records of ongoing research;
  • institutional documents and selected organizational webpages;
  • some preprints and manuscripts that have not yet entered conventional journal publication;
  • other unpublished or difficult-to-locate research materials.

These categories are not interchangeable. A doctoral dissertation may contain a complete empirical study with extensive methodological documentation, while a conference abstract may provide only a few hundred words about preliminary findings. A government report may contain original survey data, whereas another government document may simply state policy. Calling all of them grey literature does not make them equivalent forms of evidence.

Grey literature is not necessarily unpublished literature

One of the easiest mistakes is to use grey literature and unpublished research as synonyms. They overlap, but they are not identical.

A government report available publicly on an agency website has clearly been published in the ordinary sense of being made publicly available, even though it was not published by an academic journal. A thesis deposited in a university repository is similarly accessible to readers. Conversely, some genuinely unpublished studies may exist only in trial registries, institutional records, researcher files, or communications with investigators.

Grey literature Defined mainly by being outside conventional academic or commercial publishing channels; some grey literature is publicly available and formally issued.
Unpublished research Research that has not been formally published or publicly disseminated as a complete research output; it represents only part of the broader grey-literature landscape.

If your objective specifically involves studies that were never published, you may need strategies designed to identify unpublished or ongoing research rather than treating every grey-literature source as evidence of unpublished work.

Why would useful research never appear in a journal?

Journal publication is only one route through which research reaches an audience. Governments commission evaluations to inform programs. Universities archive dissertations. Organizations conduct surveys for planning or advocacy. Researchers circulate working papers before formal publication. Studies may be registered while data collection is still underway. Conference findings may never progress to full publication.

In some fields, these sources are peripheral. In others, they are central to understanding what has actually been studied or implemented. Policy, public health, education, international development, environmental research, and health technology assessment are examples of areas in which relevant evidence may be produced by organizations whose primary purpose is not academic publishing.

Why can database searching miss these materials?

Major bibliographic databases are designed primarily around particular bodies of scholarly literature. They do not comprehensively index every thesis repository, government website, institutional archive, conference program, research registry, or organizational report collection.

Grey literature can also be harder to retrieve because metadata and indexing are inconsistent. A report may have no abstract. An institutional repository may offer only basic keyword searching. A government document may move when a website is redesigned. Search interfaces may not support controlled vocabulary, sophisticated Boolean logic, bulk exporting, or reproducible search histories.

Consequently, a grey-literature search often requires several complementary approaches rather than one universal database.

Where do researchers search for grey literature?

The appropriate sources depend on the research question. JBI recommends tailoring grey-literature searching to the topic rather than searching an arbitrary list of resources. Potential routes include:

  • institutional and subject repositories;
  • thesis and dissertation collections;
  • government and intergovernmental organization websites;
  • professional association and research institute websites;
  • conference proceedings and abstract collections;
  • trial or study registries;
  • general academic search systems such as Google Scholar;
  • targeted web searching;
  • reference and citation searching;
  • contact with researchers or subject experts where justified.

The practical mechanics of finding theses, dissertations, reports, and working papers deserve separate treatment because different document types often require different repositories and search techniques.

When should grey literature be part of your search?

The strongest reason is methodological: your review question requires a sufficiently comprehensive picture of the available evidence, and relevant evidence is plausibly located outside journals.

This is particularly important in many systematic and scoping reviews. Cochrane describes searching relevant grey-literature sources such as reports, dissertations, theses, and conference abstracts as highly desirable for intervention reviews because extensive searching can reduce the risk of publication bias and identify additional relevant evidence. JBI likewise includes explicit grey-literature searching within supplementary searching for evidence syntheses.

Searching may also be valuable when:

  • your topic concerns government policy, institutional practice, programs, or implementation;
  • the field is new and journal publication may lag behind current activity;
  • your question requires theses, technical reports, evaluations, or working papers;
  • you expect completed studies with null, negative, or inconclusive results to be less visible in journals;
  • you need to identify ongoing research;
  • the review aims to map a broad evidence landscape rather than only peer-reviewed studies.

The concern about missing less visible results becomes especially consequential when excluding grey literature could alter the apparent evidence base. That possibility does not mean every grey source must be included. It means the decision should follow from the review question and protocol rather than convenience alone.

When might an extensive grey-literature search be unnecessary?

Comprehensiveness has a cost. Grey-literature searching can consume considerable time because sources are dispersed, interfaces differ, records may be poorly indexed, and screening can involve large numbers of weakly relevant documents.

A narrowly focused narrative review may not require the same search breadth as a systematic review. A project explicitly restricted to peer-reviewed journal research may legitimately exclude grey literature if that restriction fits its purpose and is reported transparently. Some questions also concern phenomena for which the relevant evidence is overwhelmingly located in conventional scholarly databases.

The decision is therefore not "grey literature is always better." A more defensible question is: What relevant evidence could exist outside the sources I am already searching, and would missing it matter to the purpose of this review?

Watch Out

Do not decide to exclude grey literature simply because it is harder to search. For reviews intended to provide a comprehensive or bias-sensitive synthesis of evidence, convenience alone is a weak methodological justification for restricting the evidence base.

Searching and including are separate decisions

Finding a grey-literature document does not automatically mean that it belongs in your review. Your eligibility criteria should determine inclusion.

If a report satisfies the predefined criteria, its publication channel alone should not substitute for appraisal. Conversely, being issued by a respected organization does not guarantee methodological rigor. The relevant question is whether the source meets the review's eligibility requirements and whether its evidence can be appropriately evaluated.

This distinction becomes particularly important with government and institutional reports used as research evidence. When eligible non-peer-reviewed material is included, it may require explicit assessment of credibility, methods, transparency, and limitations rather than an assumption that peer-review status alone determines trustworthiness.

Plan the grey-literature search before you begin

Grey-literature searching can easily become open-ended. A web search may return thousands or millions of results, and organizational websites may contain large archives with inconsistent search tools. JBI therefore recommends deciding in advance what sources will be searched, why they are relevant, and how the search will be tracked.

For a structured evidence synthesis, your protocol can specify the document types, databases or repositories, organizational websites, search terms, dates searched, and any stopping rules used for web search results. This makes the process more transparent and reduces the temptation to keep searching until the evidence happens to support a preferred conclusion.

The stopping point does not have to be identical for every project. It does need to be defensible.

04 · A Practical Example

How grey literature can change the search for evidence

Hypothetical Example

A review of a university student-support program

Suppose a researcher wants to review evidence on programs designed to improve university students' retention. A search of conventional academic databases identifies published journal studies evaluating several interventions.

Start with the research question The researcher needs evidence about whether student-support programs have been evaluated, not merely whether those evaluations became journal articles.
Identify likely evidence outside journals Universities, education agencies, research centers, and graduate researchers may have evaluated similar programs through institutional reports, dissertations, or technical documents.
Search targeted sources The researcher searches relevant repositories and institutional sources in addition to bibliographic databases, documenting the resources, terms, dates, and search limits used.
Apply the same eligibility logic A document is not included merely because it is grey literature. Each source is screened against the review's predefined inclusion and exclusion criteria.
Appraise what is found Eligible reports and dissertations are evaluated for methodological quality and limitations using criteria appropriate to their study designs and the purpose of the review.

The value of the additional search is not that grey literature is inherently superior to journal literature. It is that the publication channel should not determine which relevant studies are visible before the evidence is even assessed.

05 · What Researchers Often Get Wrong

Common misconceptions about grey literature

Misconception

Grey literature means poor-quality research

Grey literature describes a mode of production and dissemination, not a quality rating. A detailed doctoral dissertation may provide extensive methodological information, while a short organizational report may provide very little. Quality has to be assessed from the source itself and with criteria appropriate to its design.

Misconception

Grey literature means anything that was not peer reviewed

The concepts overlap but are not identical. Grey literature is usually defined in relation to conventional publishing channels. Some materials outside journals may undergo substantial institutional, editorial, technical, or academic review even though they have not undergone conventional journal peer review.

Misconception

Grey literature is the same as unpublished research

Many grey-literature documents are publicly available. Government reports, dissertations in repositories, and organizational publications may be easy to access even though they sit outside traditional journal publishing. Truly unpublished research is a narrower problem and may require searching registries, contacting investigators, or using other approaches.

Misconception

Google Scholar alone is a grey-literature search

Google Scholar can help locate some grey literature, but no single search engine comprehensively covers the category. Relevant material may reside in repositories, government websites, organizational databases, trial registries, conference collections, or other specialized sources.

Misconception

Every literature review must search every possible grey source

The search should be proportionate to the review question and methodology. A comprehensive systematic or scoping review may warrant substantial grey-literature searching, whereas another type of review may reasonably use narrower eligibility criteria. What matters is that the scope is justified rather than accidental.

Misconception

If a source was not peer reviewed, you should automatically exclude it

Peer review can be relevant when deciding what evidence to include, but it should not become an automatic proxy for methodological quality unless peer-reviewed publication is itself part of the justified eligibility criteria. If non-peer-reviewed reports are eligible, researchers should evaluate their credibility and methodological quality directly.

06 · What This Means for You

Decide whether grey literature is necessary for your review

Do not begin by compiling an enormous list of grey-literature websites. Begin with your research question and ask what forms of evidence could realistically answer it.

A simple decision framework

If you are conducting a comprehensive systematic or scoping review
Plan explicitly whether relevant grey-literature sources should be searched and document that decision in the protocol.
If governments, universities, NGOs, professional bodies, or research institutes are likely to produce important evidence on your topic
Search the relevant organizational and institutional sources rather than relying only on journal databases.
If you need evidence about emerging, ongoing, or incompletely published research
Consider registries, conference materials, repositories, preprints, working papers, and other sources appropriate to the field.
If your concern is specifically that unfavorable or inconclusive findings may be missing from journals
Consider whether searching for unpublished research is worth the additional effort for the conclusions you intend to draw.
If your review is intentionally limited to peer-reviewed journal literature
Make the restriction explicit, justify it in relation to the review's purpose, and recognize what kinds of evidence the restriction may omit.

Once you decide to search grey literature, define the boundaries before searching. Specify which source types matter, where you will look, how searches will be recorded, and when the search will be considered sufficiently complete. Grey-literature searching is difficult to reproduce perfectly, especially on the open web, but transparent documentation is still possible and methodologically valuable.

Also keep discovery separate from judgment. First determine whether a source is eligible. Then assess what confidence its methods and evidence warrant. The label "grey" should neither disqualify a source nor give it a free pass.

07 · A Quick Checklist

Before deciding whether to search grey literature

Before finalizing your search strategy, check:
Define what types of evidence are capable of answering your research or review question.
Identify whether relevant evidence is likely to exist outside conventional journal publishing.
Decide which grey-literature document types are eligible before beginning the search.
Choose repositories, registries, organizational websites, conference sources, or search systems that match the topic rather than using a generic list.
Record the sources searched, search terms used, dates searched, and any limits or stopping rules.
Apply predefined eligibility criteria to grey and conventionally published sources rather than including material merely because you found it.
Evaluate eligible evidence using appraisal criteria appropriate to its design, purpose, and source.
Report important search limitations when grey literature could not be searched comprehensively.
08 · Frequently Asked Questions

Frequently asked questions about grey literature

Is a thesis or dissertation considered grey literature?

Usually, yes. Theses and dissertations are commonly classified as grey literature because they are produced outside conventional commercial journal publishing, even though they undergo academic supervision or examination and may be publicly available through institutional or specialist repositories.

Are government reports grey literature?

They commonly are. Government reports are a major form of grey literature, although the evidential value of a particular report depends on what it contains. A report presenting a well-documented empirical study should not be treated identically to a policy statement or informational document simply because both were issued by a government agency.

Are conference abstracts grey literature?

They are commonly treated as grey literature or as part of supplementary searching in evidence synthesis. Their usefulness depends on the review question and the information available because abstracts often provide much less methodological and results detail than full papers. Whether you should include conference abstracts in a literature review therefore requires a separate eligibility decision.

Are preprints grey literature?

They may be treated as grey literature under broad definitions because they sit outside conventional peer-reviewed journal publication at the time they are posted. Classification practices vary, however, and the more important methodological issue is their publication status and how your review protocol handles them. The decision about whether to include preprints in a literature review should therefore be made explicitly.

Does grey literature have to be peer reviewed?

No. Many grey-literature sources have not undergone conventional journal peer review. Some nevertheless undergo other forms of academic, technical, institutional, or editorial scrutiny. Review status should be considered separately from the source's methodological quality and relevance.

Can I search grey literature using Google?

Yes, targeted web searching can be useful, particularly for organizational documents and reports. It should not automatically replace searches of relevant repositories, registries, institutional collections, or specialized resources. For transparent evidence synthesis, document the queries, dates, websites, and screening limits used.

Do I need grey literature for a narrative literature review?

Not necessarily. The appropriate search breadth depends on the purpose and claims of the review. If your question concerns policy, institutional practice, emerging evidence, or topics for which important information is produced outside journals, grey literature may still be valuable. If you intentionally restrict the review to peer-reviewed research, state and justify that boundary.

Can grey literature reduce publication bias?

Searching grey and unpublished sources can help identify evidence that is absent from conventional journals and may therefore reduce some risks associated with publication bias. It does not guarantee an unbiased evidence base, however. Grey literature can itself be incomplete, difficult to locate, selectively disseminated, or methodologically weak. Understanding how publication bias can distort the published literature helps explain why supplementary searching may matter.

09 · The Bottom Line

Search grey literature when the question requires evidence beyond journals

The Bottom Line

Grey literature is the broad body of research and information produced outside conventional academic or commercial publishing channels, and you should search it when relevant evidence may exist there and missing that evidence could weaken or distort your review.

The appropriate search depends on the question. A comprehensive evidence synthesis may require substantial supplementary searching, while a narrower review may reasonably set different boundaries. Define those boundaries in advance, search sources that make sense for the topic, document what you did, and evaluate the evidence you find rather than judging it solely by where it was published.

10 · Sources and Further Reading

Authoritative guidance on grey literature searching

11 · Cite this Guide

How to Cite This Guide

This guide is intended to be read, shared, and used in research, teaching, and academic work. If you draw on its ideas, explanations, or other content, please acknowledge the source by citing the guide. Doing so gives appropriate credit and helps your readers locate the original resource.

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