Grey Literature: What It Is and How to Search for It

Grey literature is the research literature that is not formally published in peer-reviewed journals — government reports, dissertations, conference abstracts, policy briefs, and organizational white papers. It is dismissed by some reviewers as second-class evidence, but that view is outdated: grey literature contains real findings, and ignoring it is one of the main ways systematic reviews end up with publication bias. This article covers what grey literature is, why to search it, and where to find it.

What counts as grey literature

The Luxembourg Convention (1997) defines grey literature as material produced "at all levels of government, academics, business, and industry, in print and electronic formats, but which is not controlled by commercial publishers." Practically, grey literature includes:

• Dissertations and theses
• Government reports (CDC, WHO, NIH, public agencies)
• Conference papers and abstracts
• NGO reports
• Working papers and pre-prints
• Policy documents and white papers
• Clinical trial registry records (for unpublished results)
• Books and book chapters (sometimes classed as grey)

Grey literature is not "bad" literature. A WHO report reviewed by 20 international experts can have stronger internal review than a single-peer-reviewed journal article.

Why to search it

Three evidence-based reasons:

1. Publication bias. Positive results are published more often and faster than null results. Excluding grey literature can inflate pooled effect sizes in meta-analyses by 10–30% (Hopewell et al., Cochrane 2007).
2. Recency. Pre-prints and conference abstracts can be 1–2 years ahead of journal publication.
3. Completeness. Some fields — policy, implementation, qualitative research — have substantial literature outside peer-reviewed journals.

PRISMA 2020 explicitly encourages grey literature searches, and Cochrane requires them for intervention reviews. Scoping reviews (see our scoping review post) treat grey literature as standard.

Where to search

A core list for most health and social science reviews:

• Google Scholar. Indexes both published and grey literature; use the first 100–200 results as a screening set.
• OpenGrey (successor archived; use the alternatives below).
• ProQuest Dissertations and Theses Global. Largest thesis repository.
• EThOS (British Library). UK theses.
• DART-Europe. European theses.
• ClinicalTrials.gov. Registered trials, many with results posted.
• WHO ICTRP. International clinical trial registry meta-search.
• NICE, Cochrane, and AHRQ websites. Systematic reviews and evidence summaries.
• CDC, WHO, government statistical offices. Public health reports.
• OAIster, BASE, Core. Academic open-access aggregators.

For policy and implementation reviews, add:

• Organizational websites (IOM, World Bank, UN agencies)
• Funding bodies' final report repositories (NIH RePORTER, UKRI Gateway)
• Think-tank libraries (RAND, Brookings, IFS)

How to search grey literature systematically

Grey literature searches are less structured than database searches — but they must still be documented to PRISMA-S standards.

1. List target sources. Decide before searching which grey literature sources you will consult. Base the list on your topic.
2. Use simplified strategies. Grey literature search interfaces are weaker than database interfaces. Run 2–4 key concept terms, not your full Boolean strategy.
3. Cap screening depth. For Google Scholar, screen the first 200 records. State the cap in your methods.
4. Log every search. Source, date, terms, number screened, number included. See our search documentation template.
5. Apply the same inclusion criteria. A grey literature document meets the same criteria as any other record.

Hand-searching

In addition to searching databases, hand-search:

• Reference lists of included studies (backward citation chasing)
• Citing papers of included studies via Web of Science, Scopus, or Google Scholar (forward citation chasing)
• Key journals' tables of contents for the last 1–2 years
• Key authors' profiles on Google Scholar or ORCID

Citation chasing commonly adds 5–15% of the final included set — the last relevant studies your database search missed. Use a citation network tool to map relationships across papers as you chase.

Contacting experts

For areas with lots of unpublished or in-progress work, contact topic experts:

• Authors of the most-cited papers in your field
• Conference presenters in the last 2 years
• Members of relevant scientific societies

A short email ("I am conducting a systematic review of X. Are you aware of any completed or in-progress work that might be relevant?") often surfaces pre-prints, unpublished theses, or forthcoming publications.

Quality assessment for grey literature

Grey literature cannot always be appraised with the same tools as peer-reviewed studies. Practical approaches:

• Use the AACODS checklist (Tyndall, 2010): Authority, Accuracy, Coverage, Objectivity, Date, Significance
• For theses, use the standard study-design tool (RoB 2, ROBINS-I, JBI, etc.)
• For organizational reports, assess methodology transparency explicitly

Do not exclude grey literature just because it is grey — appraise it on its methods.

Documentation

PRISMA-S requires you to report grey literature searches with the same rigor as database searches:

• Source name and URL
• Date searched
• Search terms used
• Number of results
• Number screened (if capped)
• Number included

Include this in a supplementary table.

Five grey literature search mistakes

1. Searching only Google (without Google Scholar, OpenGrey alternatives, or thesis databases)
2. Screening an unspecified depth of Google Scholar results — always cap and state the cap
3. Not applying inclusion criteria consistently to grey items
4. Excluding a relevant thesis because it is unpublished
5. Not documenting the grey search in the methods

The pay-off

Grey literature usually adds 5–20% of the final included set. More importantly, its inclusion reduces publication bias in the pooled estimate — which is the whole point of systematic review. Skipping it is a shortcut the discussion section will have to apologize for.