Grey literature refers to research output produced outside commercial or peer-reviewed academic publishing — theses, trial registrations, conference abstracts, preprints, and government or NGO reports. This page covers a narrower question than what grey literature is: how to actually search for it, translate a search strategy across sources that mostly do not support Boolean syntax, and document that search so it satisfies PRISMA reporting requirements for a systematic review.
Why a systematic review search plan has to include grey literature
A systematic review that searches only commercial databases (MEDLINE, Embase, Scopus, Web of Science) systematically under-represents a specific category of evidence: studies with null or unfavorable results, which are less likely to be submitted to and accepted by peer-reviewed journals in the first place. This is publication bias, and it is one of the reasons the PRISMA 2020 reporting standard explicitly requires reviewers to search sources beyond bibliographic databases — trial registries and other grey-literature channels — specifically to reduce the influence of unpublished or non-indexed studies on a synthesis’s conclusions. Cochrane Handbook guidance takes the same position: a comprehensive search is one of the features that distinguishes a systematic review from a narrative one, and comprehensiveness means going past what a database subscription indexes.
In practice this means a grey-literature search is not optional supplementary work done if time allows. For most systematic-review protocols and for Cochrane Reviews specifically, it is part of what a complete, defensible search strategy is expected to include, and its absence (or its presence without documentation) is a common reason reviewers and editors send a submission back for revision.
Where to search, by source type
There is no single index that covers grey literature the way a bibliographic database covers journal articles. Each source type below needs to be searched separately, and each has its own convention for what to record in your methods section.
| Source type | Where to search | What to document for PRISMA reporting |
|---|---|---|
| Theses & dissertations | ProQuest Dissertations & Theses Global (subscription); national/institutional repositories aggregated through BASE, CORE, or a directory like OpenDOAR; individual university repositories for institution-specific coverage. | Database/platform name, date searched, exact search string or terms used, any date or language restrictions applied, number of records retrieved. |
| Clinical trial registries | ClinicalTrials.gov and the WHO International Clinical Trials Registry Platform (ICTRP), which aggregates registries worldwide, including trials that never reported results. | Registry name, date searched, search terms/condition and intervention fields used, number of registered trials identified, number with no linked publication (relevant to a risk-of-bias/publication-bias assessment). |
| Conference abstracts & proceedings | The sponsoring professional society’s own site or archive when proceedings are not indexed elsewhere; some fields have a dedicated abstract database (e.g. major medical specialties often index recent-year conference abstracts in Embase). | Conference name(s) and years searched, whether abstracts, posters, or full proceedings were in scope, number of relevant abstracts identified, and how many were later matched to a full-text publication. |
| Preprints | Subject-specific servers — arXiv, bioRxiv, medRxiv, SSRN, ChemRxiv — searched individually, or via an aggregator that indexes several at once. | Server(s) searched, date searched, search string, number retrieved, and a note on how a preprint that was later published in a peer-reviewed journal was handled (cite the preprint, the published version, or both, and say which). |
| Government & NGO reports | National and regional government publication portals, the OECD iLibrary, the World Bank Open Knowledge Repository, and equivalent agency repositories (e.g. WHO technical report series, GAO reports) relevant to the review’s topic and geographic scope. | Each portal/agency searched, date searched, search terms, and number of reports retrieved and screened. |
| Subject-specific grey-literature tools | Structured search guides built for a discipline, such as CADTH’s Grey Matters checklist for health technology assessments, which walks a searcher through a standard set of health-related grey-literature sources in sequence. | Note which structured guide/checklist was followed, the version or date used, and which of its recommended sources were actually searched. |
General academic search engines — Google Scholar in particular — index a meaningful amount of grey literature (theses, reports, some preprints) alongside peer-reviewed content, and are often used as a supplementary check across all of the categories above rather than a replacement for searching each source type directly.
Translating a search strategy across sources that don’t support Boolean syntax
A search strategy built with Boolean operators for a bibliographic database rarely transfers directly to a grey-literature source. Most trial registries, repository search interfaces, and government portals support only basic keyword matching, sometimes with phrase-quoting but usually without nested AND/OR/NOT logic or field-specific tags ([tiab], [mesh], and similar tags). Two practical consequences follow:
- Break a complex database query into simpler component searches. Where a database search might combine population and intervention terms in one Boolean string, a grey-literature interface with no Boolean support usually needs that run as several separate keyword searches, then the results manually combined and deduplicated.
- Expect to browse, not just search. Some sources (conference proceedings pages, agency report indexes) have no search box at all, or one that only searches metadata, not full text. For these, plan for structured browsing — by year, topic category, or publishing unit — and document that browsing process the same way you would document a search string.
Because interfaces vary this much, capture a screenshot or export/save the results list (where the source allows it) at the time of searching. Many grey-literature sources are not static in the way an indexed database snapshot is — a government portal can restructure or a registry record can be updated — so the result set you screened may not be exactly reproducible later without your own saved record of it.
Documenting the search: what PRISMA-S expects
PRISMA-S is the dedicated extension to the PRISMA statement for reporting literature searches in systematic reviews (Rethlefsen et al., Systematic Reviews, 2021), developed specifically because search-methods reporting in systematic reviews was found to be inconsistent and often too sparse to reproduce. It sets out 16 reporting items covering, among others: the information sources searched (databases, registries, grey-literature sources, and how each was accessed), the full search strategies for at least one database presented so they can be rerun, any limits or restrictions applied (date, language, publication status), the total number of records identified per source, and how records were managed and deduplicated. A grey-literature search should be reported to the same standard as a database search — naming each source, the date it was searched, the terms or process used, and the number of results — not folded into a single vague sentence like “grey literature was also searched.”
That reporting then feeds the PRISMA 2020 flow diagram, which separates the Identification phase into a “databases and registers” arm and, when applicable, an “other methods” arm for records identified through grey-literature searching, citation searching, or expert contact — each arm reported with its own count of records identified, duplicates removed, and records screened. For the full 27-item PRISMA 2020 checklist and how the flow diagram fits into a complete systematic review write-up, see the PRISMA and systematic review methodology guide.
Screening and appraising grey-literature results
Grey-literature records generally go through the same two-stage screening process as database results — title/abstract screening against pre-specified eligibility criteria, then full-text screening of anything retained — but two things differ in practice. First, many grey-literature sources return records with far less structured metadata than a database export (no consistent abstract field, inconsistent author/date formatting), which makes screening slower and harder to batch through reference-management software. Second, because a grey-literature document was not peer-reviewed, reviewers commonly apply an explicit quality- or credibility-appraisal step before including it as evidence, rather than treating inclusion as equivalent to a peer-reviewed source by default. The AACODS checklist (Authority, Accuracy, Coverage, Objectivity, Date, Significance), developed by Jess Tyndall at Flinders University, is a commonly used tool for this appraisal step specifically because it was built for grey literature rather than adapted from a tool designed for peer-reviewed studies.
Frequently asked questions
Is a grey-literature search required for every systematic review?
PRISMA 2020 requires reporting whether and how sources beyond bibliographic databases were searched, but whether a grey-literature search is warranted depends on the review’s topic and the risk of publication bias in that field. For most health, social-science, and policy-relevant topics, reviewers and funders (and Cochrane, for Cochrane Reviews specifically) generally expect it; a protocol should state the decision and the reasoning either way rather than leaving it unaddressed.
How is a grey-literature search different from a database search strategy?
The core difference is interface support: bibliographic databases generally support Boolean and field-specific syntax that can be translated in structured, reportable form (see Boolean search operators for literature searching), while most grey-literature sources support only basic keyword search or structured browsing, which changes both how the search is run and how it needs to be documented.
Do grey-literature results need to go through the same PRISMA flow diagram as database results?
Yes, though they are typically tracked as a separate arm within the diagram’s Identification phase (commonly labeled “other methods” alongside “databases and registers”) so the count of records from each source type stays distinct and traceable through screening and inclusion.
What is PRISMA-S and is it required?
PRISMA-S is a 16-item extension to PRISMA specifically for reporting search methods in detail — sources searched, full search strategies, limits applied, and record counts. It is not universally mandated by every journal the way the base PRISMA 2020 checklist increasingly is, but many systematic-review methodologists and some journals recommend or require it, and following it is good practice regardless of a specific journal’s policy.
Should grey literature be appraised differently from peer-reviewed studies before inclusion?
Most reviewers apply an explicit credibility-appraisal step, such as the AACODS checklist, before treating a grey-literature document as evidence, precisely because it has not passed through journal peer review. This does not mean automatically excluding it — it means documenting how its credibility was assessed.
