- Gray literature is research, evidence, and information produced and distributed outside traditional peer-reviewed journals or academic books.
- Common types include government reports, policy briefs, theses, clinical trial registries, conference proceedings, and technical reports.
- Searching grey literature is a methodological requirement in systematic, integrative and scoping reviews to reduce publication bias and selection bias.
- Evaluation is essential: grey literature lacks peer review, so quality appraisal tools such as AACODS and DARTS must be applied before inclusion.
What Is Gray Literature?
Gray literature is research, evidence, and information produced and distributed outside traditional commercial and academic publishing channels. This means it is not published in peer-reviewed journals or commercial books.
Main Types of Grey Literature
Grey literature spans dozens of document types. The major categories, their producers, and their relevance to research are summarized below.
| Category | Document Types | Typical Producers | Key Use Cases |
| Policy and Government | Policy briefs, white papers, legislative documents, regulatory reports | Government ministries, regulatory agencies, intergovernmental organizations | Evidence for policy reviews; legal and regulatory contexts |
| Technical and Scientific | Technical reports, standards, patents, laboratory notebooks | Research institutes, national labs, standards bodies (e.g., ISO, ANSI) | Engineering reviews; technology assessment |
| Health and Clinical | Clinical trial registries, public health bulletins, surveillance reports | WHO, CDC, NHS, health departments | Systematic reviews; clinical guideline development |
| Academic and Institutional | Theses, dissertations, preprints, conference papers, working papers | Universities, academic consortia, preprint servers (arXiv, SSRN) | Methodology development; emerging research tracking |
| Non-Governmental Organizations | Program evaluations, advocacy reports, statistical compilations | NGOs, think tanks, charities, foundations | Social policy research; global development reviews |
| Industry and Corporate | Annual reports, market research, internal evaluations, product assessments | Corporations, industry associations, consultancies | Economic analyses; technology landscape reviews |
| Environmental and Ecological | Environmental impact assessments, survey reports, conservation plans | Environmental agencies, conservation bodies, field stations | Ecological meta-analyses; conservation evidence reviews |
Why Is Gray Literature Important in Research?
Grey literature is important because it addresses two of the most persistent threats to evidence integrity: publication bias and the file-drawer problem. Studies with statistically significant or positive results are approximately three times more likely to be published in peer-reviewed journals than studies with null or inconclusive findings. This asymmetry means that a review relying only on journal articles may systematically overestimate the effectiveness of an intervention.
Value of gray literature
- Currency: government and non-profit reports are often published months or years before equivalent academic literature appears.
- Breadth: grey literature covers populations, interventions, and settings that academic journals rarely prioritize, particularly in low-income countries and marginalized communities.
- Practice-based evidence: field practitioners document real-world implementation data that controlled research often misses.
- Policy relevance: policy briefs and regulatory documents reflect the translated, applied version of science, which is essential for reviews aiming to inform decision-making.
- Unpublished trials: clinical trial registries and conference abstracts may contain results from trials that were never submitted for publication, particularly when outcomes were unfavorable to a sponsor.
- Ecological and environmental data: survey reports, species inventories, and environmental impact assessments provide critical baselines for conservation and ecological meta-analyses.
How Do You Find Grey Literature Effectively?
Finding grey literature requires a multi-pronged strategy. No single database or search engine indexes all grey literature, so a systematic approach must combine purpose-built databases, organizational website searches, general search engines, and citation tracking.
The following approaches are recommended for comprehensive grey literature searching:
- Organizational website searching: Identify organizations likely to produce relevant documents (government ministries, international agencies, professional associations, leading NGOs) and search their publication or document repositories directly.
- Domain-restricted searching: Use Google with site: operators to restrict results to specific domains (e.g., site:.gov or site:who.int) combined with your key terms.
- Handsearching: Browse the publications lists of key organizations, particularly where indexing is incomplete or where document metadata is unreliable.
- Citation tracking: Check references in included grey documents to identify additional grey sources. Also check which peer-reviewed papers cite key grey documents.
- Expert and author contact: Contact researchers or organizations working in the field to ask about unpublished reports, in-progress studies, or documents not yet posted online.
- Preprint servers: Search arXiv, bioRxiv, medRxiv, SSRN, and similar platforms for preprints that may never be formally published or whose published version appeared after your search date.
Top sources of gray literature
|
Source / Platform |
Coverage |
Best For |
Access |
|
BASE (base-search.net) |
Hundreds of millions of records harvested from open-access repositories worldwide; document-type filters isolate reports, theses, and conference objects |
Broad multidisciplinary and multilingual grey literature |
Free |
|
OpenAIRE Explore (explore.openaire.eu) |
European institutional and thematic repositories, with outputs linked to projects and funders |
European grey literature and funder-mandated deposits; pairs well with BASE rather than substituting for it |
Free |
|
CORE (core.ac.uk) |
Full-text aggregation across open repositories, with API access |
Retrieving the document itself rather than a metadata record; helps mitigate link rot |
Free |
|
Grey Matters (greymatters.cda-amc.ca) |
Curated checklist of HTA agencies, regulators, guideline developers, and health economics sources, maintained by Canada’s Drug Agency |
Health technology assessment, regulatory, and clinical guideline searching |
Free |
|
WHO ICTRP (trialsearch.who.int) |
Cross-registry search spanning ClinicalTrials.gov, ISRCTN, and national registries in China, India, Japan, Brazil, and the EU |
Comprehensive trial identification in a single query, including unpublished and ongoing studies |
Free |
How Should Grey Literature Searches Be Documented?
Documentation of grey literature searching is a methodological requirement and a cornerstone of reproducibility. Every source searched, the terms used, the date of searching, the number of results, and the screening decisions must all be recorded.
| Element to Record | Example / Notes |
| Source name | Policy Commons; WHO IRIS; R Discovery |
| Date of search | 2025-03-15 |
| Search terms used | ‘mental health’ AND ‘adolescents’ AND ‘low income’ |
| Filters applied | Date: 2015 to present; Language: English |
| Number of results returned | 247 |
| Number screened for relevance | 38 |
| Number included after screening | 11 |
| Reason for exclusion (summarized) | Not relevant to population (n=18); no methods described (n=9) |
How Do You Evaluate Grey Literature?
Because grey literature does not undergo peer review, quality varies widely and critical appraisal is essential before any grey document is included in a review. Evaluation serves two purposes: determining whether a document is reliable enough to include, and extracting information about potential bias that should be noted in the review.
The AACODS Framework
The AACODS checklist was developed specifically for critical appraisal of grey literature in health and social science contexts. It provides a structured, reproducible evaluation across six criteria.
| Criterion | Evaluation Questions | Score (0-2) | Notes |
| Authority | Is the producing organization identified? Is it recognized as credible in the field? Is author expertise indicated? | ||
| Accuracy | Is the methodology clear and reproducible? Are data sources identified? Are conclusions consistent with the evidence presented? | ||
| Coverage | Is the scope of the document clearly defined? Is the content sufficiently comprehensive for the review question? | ||
| Objectivity | Are potential conflicts of interest disclosed? Is the language balanced rather than promotional? Are limitations acknowledged? | ||
| Date | Is a publication or last-updated date provided? Is the content current enough for the review period? | ||
| Significance | Does this document add information not available in peer-reviewed literature? Is it cited by or referenced in other key works? |
Each criterion is typically scored on a two-point scale (0: criterion not met; 1: partially met; 2: fully met), producing a maximum score of 12. There is no universal cut-off, but reviewers should apply a consistent threshold agreed upon in the protocol.
The DARTS Tool
DARTS offers a complementary and simpler framework, well suited to quick screening of large volumes of grey literature. Its five criteria are: Date, Author, References, Type, and Sponsor. DARTS is easily adapted into a spreadsheet and is useful for systematic tracking when large numbers of grey documents must be appraised quickly.
QUEST: Evaluating Online Health Information
The QUality Evaluation Scoring Tool (QUEST) uses a 28-point system designed for health information published online. It assesses Authorship (0 to 2), Attribution of references (0 to 3), Conflict of interest (0 to 2), Currency (0 to 2), and the degree to which the content supports rather than replaces professional medical advice. QUEST is most useful for systematic reviews that include patient-facing or health promotion grey literature.
Limitations of Using Grey Literature?
Grey literature provides important evidence, but its use introduces methodological challenges that reviewers must anticipate and manage.
Inconsistent Indexing and Discoverability
Grey literature is distributed across thousands of organizational websites, repositories, and platforms without consistent metadata or indexing. A single government report may exist in multiple versions on multiple pages without a stable URL. This makes comprehensive searching resource-intensive and difficult to replicate exactly.
Variable Quality and Reliability
Without peer review, grey literature quality ranges from rigorously conducted independent evaluations to marketing documents produced to support a commercial agenda. Reviewers must invest time in quality appraisal for every document, rather than relying on journal selection as a proxy for minimum quality standards.
Language Barriers
Grey literature is published in hundreds of languages, and search engines do not reliably surface non-English documents. Restricting a review to English-language grey literature introduces language bias, particularly for topics where major evidence is produced in French, Spanish, Arabic, Chinese, or Portuguese-language policy environments.
Link Rot and Inaccessibility
Grey literature documents are frequently moved, updated without version control, or removed entirely from websites. A report cited in 2020 may be inaccessible by 2025. Reviewers should archive documents at the point of retrieval (using tools such as the Internet Archive’s Wayback Machine) and note access dates in citations.
Practical Strategies for Managing These Challenges
- Set a defined search date and archive all retrieved documents on that date.
- Define inclusion and exclusion criteria for grey literature in the protocol before searching begins.
- Use persistent identifiers (DOIs, handle.net links) where available.
- Build a contact strategy: email organizations directly when key documents are unavailable online.
- Consult a librarian with systematic review experience to design the grey literature search strategy.
Patents as Grey Literature
Are patents considered grey literature?
Yes. Patents are a well-established category of grey literature. They are produced and published outside commercial academic journals, issued by government-affiliated patent offices, and distributed through specialized registries rather than conventional publishing channels.
Why are patents relevant to academic and evidence synthesis research?
Patents document novel inventions, processes, and formulations at the point of creation, often years before equivalent findings appear in peer-reviewed journals. In fields such as pharmacology, biotechnology, materials science, and medical devices, the patent literature captures the cutting edge of applied research. A systematic or scoping review in these areas that excludes patents may miss foundational technical evidence about how an intervention works, how it was developed, or whether its underlying mechanism has been independently validated.
Where can patents be searched?
The major freely accessible patent databases include:
- Espacenet (European Patent Office): covers patents from over 100 countries
- Google Patents: broad international coverage with full-text search
- USPTO Patent Full-Text Database: US patents from 1976 onward
- WIPO PATENTSCOPE: international applications filed under the Patent Cooperation Treaty
- J-PlatPat: Japanese patents in English translation
Grey Literature in Different Literature Reviews
Gray literature in systematic reviews
Cochrane, PRISMA 2020, and the Joanna Briggs Institute (JBI) all recommend grey literature searching as a standard component of systematic reviews. Failing to search grey literature introduces selection bias and threatens the validity of any quantitative synthesis. Reviewers must document every grey literature source searched, regardless of whether it yielded included studies.
In practice, a systematic review of a pharmaceutical intervention should at minimum search relevant clinical trial registries, regulatory agency databases (such as the FDA or EMA), and conference proceedings from key specialty societies. A systematic review of a public health program should also include government evaluations, agency reports, and NGO documentation.
Grey Literature in Scoping Reviews
Scoping reviews explicitly aim to identify what types of evidence exist, which populations have been studied, and where gaps remain. Excluding grey literature in a scoping review would systematically underrepresent practice-based knowledge, organizational evidence, and evidence from settings that do not produce peer-reviewed publications.
The PRISMA-ScR extension and the JBI scoping review methodology both specify that grey literature sources should be searched as part of the standard scoping review protocol, with sources documented in an appendix.
Grey Literature in Integrative Reviews
Integrative reviews deliberately combine evidence from diverse sources: experimental and non-experimental studies, theoretical papers, clinical guidelines, and grey literature. This breadth is the defining strength of the integrative approach, and it makes grey literature not merely useful but structurally central to the method.
Frequently Asked Questions
Does grey literature count as a legitimate source in academic work?
Yes, grey literature is a legitimate and often essential source in academic research, particularly in evidence synthesis. Review methodologists, including Cochrane, JBI, and PRISMA, explicitly require it. What matters is transparent appraisal of quality and clear documentation of how the source was found and why it was included.
How much grey literature is too much in a systematic review?
There is no prescribed ratio of grey to peer-reviewed literature. Inclusion should be driven entirely by relevance and quality, not by volume. Some systematic reviews include dozens of grey documents; others include none after screening. What matters is that the search for grey literature was comprehensive and documented, regardless of how many documents were ultimately included.
Can I use Wikipedia as grey literature in a review?
Wikipedia is generally not considered an appropriate grey literature source for systematic or scoping reviews. It is an encyclopedic, community-edited source without authorial accountability, citable methods, or verifiable provenance. It may be useful for background orientation or identifying key terms, but should not be cited as evidence in an evidence synthesis.
How do I handle grey literature that is only available in a language I do not speak?
Language limitations should be acknowledged in the review limitations section. If resources allow, machine translation or professional translation is preferable to exclusion. For some topics, particularly global health and international policy, excluding non-English grey literature introduces a significant language bias that can skew conclusions. Some review teams specifically recruit multilingual reviewers to address this.
Do I need to do a grey literature search for a rapid review?
Rapid reviews involve deliberate scope restrictions to reduce time, and grey literature searching is often abbreviated. However, the decision to limit or omit grey literature searching must be explicitly stated and justified in the methods section. Readers and decision-makers need to know that the evidence base may be incomplete as a result of the rapid approach.
How do I report grey literature in my PRISMA flow diagram?
Grey literature sources are typically reported in the PRISMA flow diagram as separate streams from database searches. Each source (for example, Policy Commons, WHO IRIS, and organizational websites) should be listed with the number of records identified, screened, and included. The PRISMA 2020 update provides an updated flow diagram template that accommodates multiple sources.
How can I find gray literature for free?
Comprehensive grey literature searching does not require paid subscriptions. Google Scholar, direct organizational website searches using domain-restricted Google queries, open-access repositories (OpenDOAR, CORE, BASE), clinical trial registries, and preprint servers are all freely accessible. A number of free aggregators, including the WHO IRIS and the ERIC database, also provide extensive grey literature. Consulting your institutional librarian can also reveal free alternatives or interlibrary loan options.
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