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Grey Literature: Definition, Meaning & Examples | CASRAI

Grey literature refers to research and information produced outside of traditional commercial publishing and peer-reviewed journal channels. It encompasses government and policy reports, technical documents, working papers, dissertations and theses, conference papers, preprints, and statistical datasets — outputs that may not appear in standard bibliographic databases but are nonetheless significant sources of evidence. Grey literature is especially important in systematic reviews, where excluding it introduces publication bias.

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Defining and classifying grey literature

The most widely cited definition in health and social sciences research comes from the Luxembourg Convention on Grey Literature (1997), which defines grey literature as "that which is produced on all levels of government, academics, business and industry in print and electronic formats, but which is not controlled by commercial publishers." The scope of the term has expanded significantly in the internet age to include datasets, preprints, blog posts from research institutions, policy briefs, slide decks from conferences, and social media threads from scientists. For systematic review purposes, the most operationally useful definition focuses on unpublished or non-indexed outputs: documents that may contain relevant research findings but would not appear in a search of PubMed, Scopus, or Web of Science alone. This includes theses and dissertations, clinical trial registries, government statistical releases, and reports from charities and NGOs.

Key sources for finding grey literature

Because grey literature is by definition not systematically indexed, finding it requires targeted searching of specialist sources. The British Library's EThOS (Electronic Theses Online Service, available at ethos.bl.uk) provides access to UK doctoral theses, while ProQuest Dissertations and Theses covers North American and international institutions. Government portals — including GOV.UK, the European Commission's publications office, and the WHO Institutional Repository — are essential for policy and regulatory documents. Clinical trial registries including ClinicalTrials.gov, the ISRCTN registry, and the EU Clinical Trials Register hold records of registered studies regardless of whether results were published. OpenGrey, which provided a searchable database of European grey literature through the SIGLE (System for Information on Grey Literature in Europe) dataset, was discontinued as a live service on 1 December 2020. Its data was archived in the DANS EASY repository and remains downloadable, but researchers now rely on direct searching of national portals, institutional repositories, and specialist databases.

Grey literature and publication bias in systematic reviews

Publication bias is the tendency for studies with statistically significant or positive results to be more likely to be published in peer-reviewed journals than studies showing no effect or negative outcomes. This creates a skewed evidence base: if a systematic review searches only indexed journals, it will capture a disproportionate share of positive findings. Grey literature frequently contains null results — because government agencies, regulatory bodies, and clinical researchers are often required to document all findings regardless of outcome. The PRISMA 2020 statement (Preferred Reporting Items for Systematic reviews and Meta-Analyses) requires systematic review authors to document their grey literature search strategy in detail, including which sources were searched, the dates of searches, and what terms were used. Failing to search grey literature is now regarded as a methodological weakness that reviewers and journal editors are likely to flag.

Grey literature in practice: challenges and quality assessment

Searching grey literature is more time-consuming than database searching and requires researcher judgement about source selection, because there is no single grey literature equivalent of PubMed. Researchers supplement structured database searching with hand-searching of organisational websites, emailing study authors and relevant professional bodies, and searching preprint servers such as SSRN and medRxiv. Quality assessment of grey literature is also more challenging than for peer-reviewed articles: reports may not describe their methods transparently, may represent the interests of the commissioning body, or may not have been independently reviewed. Standard tools such as CASP (Critical Appraisal Skills Programme) checklists can be adapted for grey literature appraisal, and PRISMA guidance recommends that inclusion and quality assessment criteria are pre-specified in the review protocol. Despite these challenges, the consensus among systematic reviewers is that the benefits of including grey literature — primarily the reduction of publication bias — outweigh the additional effort.

Key facts

At a glance

Definition: Research produced outside traditional commercial peer-reviewed publishing channels
Examples: Government reports, theses, working papers, clinical trial registrations, preprints, NGO reports
OpenGrey/SIGLE: European grey literature database discontinued 1 December 2020; data archived in DANS EASY
EThOS: British Library's Electronic Theses Online Service; principal UK thesis repository (ethos.bl.uk)
Systematic reviews: PRISMA 2020 requires documented grey literature search strategy
Publication bias: Grey literature often contains null and negative findings absent from indexed journals

Common misconceptions

What people often get wrong

Often heard: Grey literature is low-quality or unreliable.

Actually: Grey literature includes government statistical reports, regulatory submissions, and clinical trial data — rigorous evidence that has not passed through commercial peer review but is not therefore untrustworthy. Quality must be assessed on a case-by-case basis.

Often heard: Systematic reviews only need to search peer-reviewed databases.

Actually: PRISMA 2020 requires a documented grey literature search. Excluding grey literature introduces publication bias, potentially skewing the review's conclusions towards positive results.

Often heard: OpenGrey is still available for searching.

Actually: OpenGrey was discontinued on 1 December 2020. Its data was archived in the DANS EASY repository and can be downloaded, but it is no longer a live searchable service.

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