Should You Search Grey Literature More Extensively When Evidence Is Scarce?

If databases find very little, should you search much further?

You search the major bibliographic databases carefully and find only a handful of relevant studies. At that point, the scarcity itself raises another question: is the research genuinely scarce, or is some of it simply difficult to find?

Dissertations, theses, conference materials, research reports, study registries, regulatory documents, institutional repositories, and other materials outside conventional journal publishing may contain relevant evidence. Some may describe completed studies that never became journal articles. Others may reveal ongoing studies, unpublished outcomes, or additional information about studies you already know.

When the published literature is sparse, finding even one additional relevant study can materially alter your understanding of the evidence base. That makes grey literature particularly worth considering, but it does not mean searching everywhere indefinitely.

Sparse evidence usually strengthens the case for searching beyond journals

Grey literature can change what “very little evidence” actually means

Grey literature is a broad term for research and other information produced outside traditional commercial or academic journal publishing. Depending on the field and research question, it may include dissertations and theses, conference abstracts and proceedings, government and organizational reports, technical reports, study registries, regulatory documents, institutional repository materials, and other forms of research dissemination.

Cochrane recommends searching relevant grey literature sources such as reports, dissertations, theses, and conference abstracts as part of efforts to identify evidence comprehensively and reduce the risk of publication bias. It also identifies trial registers and other unpublished-data sources as important components of systematic searching.

This becomes especially consequential when only a few relevant studies are known. Cochrane notes that unpublished studies may have greater potential to affect conclusions in situations where few relevant studies exist.

Published literature is not the same thing as all research that has been conducted

A study can be completed without producing a conventional journal article. Results may remain in a dissertation, conference abstract, registry record, government report, sponsor document, or institutional repository. A registered study may also have ended without an easily identifiable publication.

This creates a basic distinction:

The first finding does not establish the second.

This distinction is one reason a claim that very little evidence exists becomes considerably more convincing after researchers have looked beyond the easiest-to-search publication channels.

Publication processes can distort the visible evidence base

Studies do not have equal probabilities of reaching conventional publication. Results, study size, investigator decisions, sponsor decisions, editorial processes, and other factors can influence dissemination.

Consequently, relying exclusively on published journal articles can produce an incomplete picture of the research that was undertaken. Searching beyond journals cannot guarantee elimination of reporting bias, but it can help identify evidence that would otherwise remain invisible.

The issue becomes particularly important when the evidence base contains only a few studies. If three published studies are available and a fourth relevant unpublished study is found, that additional study represents a substantial addition to the evidence base. In a literature containing hundreds of studies, one additional record may have far less influence.

Grey literature is not one database

There is no universal grey literature source that covers every discipline and document type. The appropriate search therefore depends on what kinds of evidence your topic is likely to generate.

The table is a map, not a mandatory itinerary. A review of a pharmaceutical intervention and a review of a university teaching practice should not automatically search identical grey literature sources.

Start with sources that are structurally connected to the research question

A productive grey literature search is usually targeted rather than indiscriminate.

If the question concerns clinical trials, trial registries and regulatory sources may be high priority. For educational research, dissertations, institutional repositories, conference proceedings, government education agencies, and professional organizations may be more productive. For policy interventions, government and organizational reports may contain evidence that was never intended for journal publication.

Ask where researchers working on this topic would plausibly deposit, register, present, or report their work if it did not appear in a conventional journal.

Known studies can lead you into the less visible literature

When evidence is scarce, each known study becomes a useful retrieval node.

Check its reference list. Look at papers that cite it. Search the authors' names and research groups. Examine trial or study registration identifiers. Look for dissertations, conference presentations, protocols, reports, supplementary materials, or multiple publications associated with the same underlying study.

Cochrane requires reference-list checking for included studies and relevant systematic reviews and recommends searching previous reviews and other sources to identify additional studies.

This approach can be more productive than repeatedly adding synonyms to an already sensitive database query.

Searching grey literature does not mean accepting everything you find

Discovery and appraisal are separate processes.

A dissertation does not become weak evidence because it is a dissertation. A government report does not become strong evidence because a government published it. A conference abstract may describe a rigorous randomized trial but provide too little methodological information for confident appraisal.

Evaluate the underlying study using criteria appropriate to its design. Consider methods, risk of bias, relevance, completeness of reporting, and the certainty with which the document can be linked to the study it describes.

Conference abstracts require particular restraint

Conference abstracts can be valuable for identifying studies, especially recent or unpublished ones, but they often contain limited methodological and results information. Some eventually lead to full publications; others do not.

If an abstract appears eligible, look for a full report, registration, protocol, author manuscript, or related publication. When only the abstract is available, make clear what information could and could not be verified rather than filling gaps by assumption.

Searching harder eventually produces diminishing returns

Grey literature searching can be resource-intensive. Websites may have weak search interfaces. Repository metadata can be inconsistent. Documents may not be indexed. Search engines can return enormous numbers of poorly targeted results.

A sparse literature justifies greater effort, but not an endless search.

Prioritize sources according to the probability that they contain eligible studies and the consequence of missing those studies. Keep a record of what has already been searched and what each additional source contributes.

If repeated searches of the most plausible sources yield no new eligible evidence, further searching increasingly becomes a question of marginal value rather than methodological virtue.

Document grey literature searches so someone else can understand what you did

PRISMA 2020 asks systematic reviewers to specify databases, registers, websites, organizations, reference lists, and other sources searched or consulted, including the date each source was last searched. Its expanded guidance also calls for reporting relevant online repositories, websites, organizations, and contacts used to identify studies.

Grey literature searching is sometimes less reproducible than database searching because interfaces change and browsing may involve judgment. That makes documentation more important, not less.

Record enough detail to explain where you searched, when you searched it, how you searched it, and what restrictions were applied.

What if database searching finds only three studies?

The grey literature search was valuable because it targeted places where this particular research was likely to exist. Searching fifty unrelated websites would not necessarily have made the review more rigorous.

Grey literature searching can fail in opposite directions

Search deeper where missing evidence is most likely to be hiding

When conventional searching produces little evidence, the next step is not to search the entire internet. Map the likely research ecosystem around your question.

Grey literature searching therefore belongs within the broader process of deciding what to do when only a few relevant studies exist. It should generally precede attempts to solve scarcity by making the research question substantially broader.

If the search remains sparse after appropriate published and grey literature sources have been examined, the methodological task changes. You no longer need to make the literature look larger. You need to represent what the limited evidence can and cannot support.

Before concluding your grey literature search

Questions about searching grey literature when evidence is limited

When evidence is scarce, look beyond what was easiest to publish and index

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.