Grey literature refers to research output produced outside traditional commercial or academic publishing channels. In the context of a systematic review, grey literature includes conference abstracts, doctoral and master's theses, government and agency reports, preprints, clinical trial registries, regulatory documents, and working papers that are not indexed in databases like PubMed, Embase, or Web of Science. Including grey literature is essential for reducing publication bias, because studies with statistically significant or favorable results are more likely to be published in peer-reviewed journals, while studies with null or negative findings often remain unpublished. The Cochrane Handbook for Systematic Reviews of Interventions (Higgins et al., 2023) explicitly recommends searching for grey literature to ensure a comprehensive and unbiased evidence synthesis. Failing to search beyond standard databases risks overestimating treatment effects and producing conclusions that do not reflect the full body of available evidence.
The PRISMA 2020 guidelines (Page et al., 2021) require authors to report all information sources searched, including grey literature databases and registries. Reviewers and journal editors increasingly expect to see documented grey literature searches in systematic review manuscripts. This guide provides a practical, source-by-source approach to identifying, searching, documenting, and appraising grey literature for your next review.
Why Grey Literature Matters for Reducing Publication Bias
Publication bias is one of the most significant threats to the validity of a systematic review. Research consistently shows that studies reporting positive or statistically significant results are published at higher rates and with shorter timelines than studies reporting null or negative findings. A landmark analysis by Scherer et al. (2018) estimated that approximately 50 percent of completed clinical trials remain unpublished years after completion. When a systematic review relies exclusively on published literature, it captures a skewed sample of the available evidence.
The consequences are measurable. Meta-analyses that exclude grey literature tend to produce larger pooled effect sizes than those that include it. This inflation occurs because the missing studies, those with null results, smaller effects, or inconclusive findings, would pull the pooled estimate downward if included. By searching grey literature sources, you recover studies that were never submitted to journals, were rejected, or are still in progress. This broader net produces a more accurate estimate of the true effect.
Several tools can help you assess and visualize publication bias after incorporating grey literature. A funnel plot displays the relationship between study effect sizes and their precision; asymmetry in the funnel suggests missing studies. You can generate funnel plots using our free funnel plot generator. For a deeper explanation of statistical methods for detecting publication bias, including Egger's test and trim-and-fill analysis, see our guide to publication bias detection methods.
Beyond reducing bias, grey literature serves additional purposes. It provides access to early-stage evidence from preprints and trial registries before peer-reviewed publication. It captures research conducted by government agencies, international organizations, and regulatory bodies that may never appear in academic journals. And it identifies ongoing or recently completed studies that could influence the conclusions of your review.
Comprehensive Source List Organized by Type
The following sections organize grey literature sources into categories. Each category includes specific databases, repositories, and platforms with their URLs and coverage.
Conference Abstracts and Proceedings
Conference presentations often report preliminary results that are never developed into full publications. Searching conference abstract databases captures this evidence.
- Conference Proceedings Citation Index (CPCI): Part of Web of Science, indexes proceedings from over 12,000 conferences annually across sciences, social sciences, and humanities
- Embase Conference Abstracts: Embase indexes conference abstracts from major biomedical and pharmaceutical conferences, searchable alongside journal articles
- Individual society proceedings: Many professional societies (American Heart Association, American Society of Clinical Oncology, European Society of Cardiology) maintain searchable archives of annual meeting abstracts on their websites
- Scopus Conference Papers: Scopus indexes selected conference proceedings and allows filtering by document type
Theses and Dissertations
Doctoral and master's theses represent substantial, peer-reviewed (by committee) research that may not be published in journals.
- ProQuest Dissertations and Theses Global: The largest single repository of graduate dissertations and theses, containing over 5 million records from institutions worldwide
- DART-Europe E-Theses Portal: Aggregates open-access research theses from over 600 European universities
- EThOS (British Library): Provides access to UK doctoral theses, with over 500,000 records
- NDLTD (Networked Digital Library of Theses and Dissertations): An international organization promoting electronic theses and dissertations, linking to institutional repositories globally
- Institutional repositories: Most universities maintain digital repositories (e.g., DSpace, EPrints) where students deposit theses. Check repositories of institutions known for research in your topic area
Government and Agency Reports
Government agencies, health technology assessment bodies, and international organizations produce research reports that are often more comprehensive than journal articles.
- NICE Evidence Search: The National Institute for Health and Care Excellence (NICE) maintains a search portal covering guidance, evidence summaries, and health technology assessments relevant to UK healthcare
- Agency for Healthcare Research and Quality (AHRQ): Publishes systematic evidence reviews, technology assessments, and research reports (effectivehealthcare.ahrq.gov)
- WHO publications: The World Health Organization publishes technical reports, guidelines, and research documents through its institutional repository (apps.who.int/iris)
- Centers for Disease Control and Prevention (CDC): Publishes Morbidity and Mortality Weekly Reports (MMWR) and other technical documents
- National Technical Information Service (NTIS): U.S. government repository for federally funded research reports
Preprint Servers
Preprints are manuscripts deposited on public servers before peer review. They provide early access to research findings and capture studies that may never proceed to formal publication.
- medRxiv: Preprint server for health sciences, operated by Cold Spring Harbor Laboratory, Yale, and BMJ
- bioRxiv: Preprint server for biological sciences
- SSRN (Social Science Research Network): Covers social sciences, humanities, and applied sciences; now part of Elsevier
- arXiv: Primarily physics, mathematics, and computer science, but increasingly used for quantitative health research
- OSF Preprints: Open Science Framework aggregates preprints from multiple discipline-specific servers
- Research Square: Multi-disciplinary preprint platform with a focus on transparency
Clinical Trial Registries
Trial registries document protocols and results of clinical studies, including those that were completed but never published. Searching registries is mandatory for Cochrane reviews and strongly recommended by PRISMA 2020.
- ClinicalTrials.gov: The largest clinical trial registry, maintained by the U.S. National Library of Medicine, containing over 450,000 registered studies from 221 countries
- WHO International Clinical Trials Registry Platform (ICTRP): A meta-registry that aggregates records from national and regional trial registries worldwide, including ClinicalTrials.gov, EU Clinical Trials Register, ISRCTN, and others
- EU Clinical Trials Register: Covers clinical trials conducted in the European Economic Area
- ISRCTN Registry: International registry of clinical trials, originally focused on randomized controlled trials
- ANZCTR (Australian New Zealand Clinical Trials Registry): Covers trials conducted in the Australia and New Zealand region
Regulatory Documents
Regulatory agencies review unpublished data submitted by pharmaceutical and medical device companies. These documents often contain efficacy and safety data not available elsewhere.
- FDA (U.S. Food and Drug Administration): Medical and statistical reviews for approved drugs and biologics are available through Drugs@FDA (accessdata.fda.gov). Advisory committee briefing documents provide detailed efficacy and safety analyses
- EMA (European Medicines Agency): European Public Assessment Reports (EPARs) contain clinical data summaries, risk assessments, and scientific discussions for centrally authorized medicines (ema.europa.eu)
- Health Canada Drug Product Database: Includes Summary Basis of Decision documents with clinical evidence summaries
- TGA (Therapeutic Goods Administration, Australia): Publishes Australian Public Assessment Reports for prescription medicines
Other Grey Literature Sources
- OpenGrey: A European-focused repository for grey literature in science, technology, biomedical science, economics, social science, and humanities. Note that OpenGrey has transitioned some of its content to other platforms; check current availability
- Google Scholar: While primarily indexing published literature, Google Scholar also captures theses, preprints, conference papers, and reports that are not found in traditional databases. Use it as a supplementary search, not a primary source
- Grey Literature Report (New York Academy of Medicine): A curated collection of grey literature in public health, updated regularly
- BASE (Bielefeld Academic Search Engine): Searches over 300 million documents from more than 10,000 content providers, including many grey literature sources
- CORE: Aggregates open-access research outputs from repositories and journals worldwide