Grey Literature Search
Systematic Search and Retrieval of Grey Literature · Also known as: grey literature, gray literature, unpublished literature
Grey literature comprises documents and data not published through conventional commercial channels—including theses, government reports, clinical trial registries, conference abstracts, organizational policy documents, and working papers. Unlike journal articles, grey literature is not indexed in MEDLINE or Scopus and often lacks peer review. However, it is crucial for systematic reviews because it may contain null or negative findings that are less likely to be published (publication bias). Systematic grey literature searching is now a standard component of evidence synthesis and is recommended by the Cochrane Collaboration, PRISMA, and other methodological guidelines.
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When to use it
Search grey literature whenever you are conducting a systematic review, meta-analysis, or comprehensive literature review. It is especially important for: (1) intervention effectiveness reviews (where publication bias is greatest), (2) questions in emerging fields (where peer-reviewed literature is sparse), (3) policy or technology areas where government agencies or industry produce primary research, (4) topics of regional or local interest (where national language publications in local repositories may be missed), and (5) health technology assessments or guideline development (where regulatory submissions and clinical trial data are critical). You may minimize grey literature searching if conducting a rapid review with tight time constraints, but even then, trial registries and thesis summaries should be searched.
Strengths & limitations
- Identifies studies with null or negative findings that would otherwise remain hidden, counteracting publication bias.
- Retrieves recent findings (dissertations, government reports) before formal peer-reviewed publication.
- Accesses data from clinical trial registries that may not be published as journal articles (especially important for adverse events).
- Includes policy documents, standards, and guidelines that shape practice even if they are not research articles.
- Improves the comprehensiveness and credibility of evidence synthesis.
- Grey literature is often of lower methodological quality than peer-reviewed articles, with incomplete reporting, smaller samples, or underpowered designs.
- There is no standardized indexing system; searches must be tailored to each source's interface, making grey literature searches time-consuming.
- Access may be restricted or geographically limited (e.g., some thesis repositories require institutional affiliation).
- Retrieval may yield untraceable or unavailable documents, reducing the final pool of usable studies.
- Including low-quality grey literature in a meta-analysis can distort pooled estimates if not carefully assessed for bias.
Frequently asked
Is grey literature searching required for all systematic reviews, or only some?
Current guidelines recommend grey literature searching for all systematic reviews, especially intervention reviews where publication bias is greatest. However, the extent of grey literature searching may be proportionate to resources: a rapid review may search trial registries and a few thesis repositories, while a full Cochrane review searches comprehensively across multiple sources. Your protocol should specify which grey literature sources you will search and justify any decisions to limit searching.
How do I access dissertations if my institution does not subscribe to ProQuest?
ProQuest Dissertations & Theses is the largest source but requires institutional subscription or individual purchase. Free alternatives: EThOS (British Library, PhD theses from UK universities), national thesis servers (many countries maintain government-funded repositories; e.g., Dart-Europe for Europe), Google Scholar (often links to PDF full-texts), and contacting authors directly via email or ResearchGate. For non-English theses, search your country's national thesis database.
Should I include trial registry records (results summaries) in my meta-analysis if the full manuscript is not published?
Yes, if the registry record contains sufficient detail on methods and results (including effect size, confidence interval, and adverse events). Trial registry results are increasingly used in systematic reviews, especially for recent trials where journal publication lags. You may consider the registry results separately in sensitivity analysis (published vs. unpublished) to assess the impact on pooled estimates. Always note in your review that registry data, though detailed, typically lacks the full methodological narrative and discussion present in a journal article.
How much effort should I invest in grey literature searching, and when is it reasonable to stop?
PRISMA guidelines suggest a 'reasonably comprehensive' search without specifying exact effort. Practical guidance: search at least 2–3 major grey literature sources (theses, trial registries, a report database). Document your search strategy, terms, and result counts. Continue searching until you are seeing the same studies repeatedly (saturation). For intervention reviews, trial registries are non-negotiable. For policy or technology reviews, government and organizational databases are essential. If you are under time pressure, prioritize trial registries and local/national thesis repositories over broad web searching.
Sources
- Rothstein, H. R., & Hopewell, S. (2009). Grey literature. In J. P. Higgins & S. Green (Eds.), Cochrane handbook for systematic reviews of interventions (Version 5.0.2, Chapter 13). The Cochrane Collaboration. link ↗
- Wood, A. M., Egger, M., Gluud, L. L., Schulz, K. F., Jüni, P., Altman, D. G., & Gluud, C. (2008). Empirical evidence of bias in treatment effect estimates in controlled trials with different interventions and outcomes: meta-epidemiological study. BMJ, 336(7644), 601–605. DOI: 10.1136/bmj.39465.451748.AD ↗
- Mahood, Q., Van Eerd, D., & Irvin, E. (2014). Searching for grey literature for systematic reviews: challenges and benefits. Research Synthesis Methods, 5(4), 353–364. DOI: 10.1002/jrsm.1106 ↗
How to cite this page
ScholarGate. (2026, June 3). Systematic Search and Retrieval of Grey Literature. ScholarGate. https://scholargate.app/en/research-skills/grey-literature-search
Which method?
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