Grey Literature in Meta-Analyses
Abstract
Background In meta-analysis, researchers combine the results of individual studies to arrive at cumulative conclusions. Meta-analysts sometimes include "grey literature" in their evidential base, which includes unpublished studies and studies published outside widely available journals. Because grey literature is a source of data that might not employ peer review, critics have questioned the validity of its data and the results of meta-analyses that include it. Objective To examine evidence regarding whether grey literature should be included in meta-analyses and strategies to manage grey literature in quantitative synthesis. Methods This article reviews evidence on whether the results of studies published in peer-reviewed journals are representative of results from broader samplings of research on a topic as a rationale for inclusion of grey literature. Strategies to enhance access to grey literature are addressed. Results The most consistent and robust difference between published and grey literature is that published research is more likely to contain results that are statistically significant. Effect size estimates of published research are about one-third larger than those of unpublished studies. Unfunded and small sample studies are less likely to be published. Yet, importantly, methodological rigor does not differ between published and grey literature. Conclusions Meta-analyses that exclude grey literature likely (a) over-represent studies with statistically significan…
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Publication bias occurs with positive results being more likely to be published.
“As publication bias occurs with positive results being more likely to be published (Conn et al., 2003), the possibility of publication bias affecting the result of the meta-analysis was also investigated.”
From The Foreign Language Classroom Anxiety Scale and Academic Achievement: An Overview of the Prevailing Literature and a Meta-analysis (2020) · cites this paper as Conn et al., 2003
The inclusion of grey literature has been found to negate the effects of publication bias in meta-analyses.
“The inclusion of grey literature has been found to negate the effects of publication bias in meta-analyses (Conn, Valentine, Cooper, H, & Rantz, 2003).”
From The Foreign Language Classroom Anxiety Scale and Academic Achievement: An Overview of the Prevailing Literature and a Meta-analysis (2020) · cites this paper as Conn, Valentine, Cooper, H, & Rantz, 2003
To further reduce potential bias, additional search strategies were applied, including contacting experts, citation searching, searching reference lists, and searching preprint, clinical register, and conference databases.
“To further reduce potential bias, additional search strategies were applied, and gray literature not previously peer-reviewed (see Conn et al., 2003) and unpublished information were identified.”
From Smartphone-delivered mental health care interventions for refugees: A systematic review of the literature (2023) · cites this paper as Conn et al., 2003
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References 39 · Cited by 400
- Authors
- Conn, V. S., Valentine, J. C., Cooper, H. M., & Rantz, M. J.
- Affiliations
- University of Missouri (US)
- Venue
- Nursing Research
- Published
- 2003-07
- Volume
- 52
- Issue
- 4
- Pages
- 256-261
- ISSN
- 0029-6562 (ISSN-L)
- OpenAlex
- W2137020879
- Cited by
- 400
- References
- 39
- Citations by year
- 2026: 13 · 2025: 23 · 2024: 26 · 2023: 32 · 2022: 46 · 2021: 23 · 2020: 27 · 2019: 25 · 2018: 17 · 2017: 24 · 2016: 15 · 2015: 11 · 2014: 18 · 2013: 17 · 2012: 8
- FWCI
- 1.76
- Type
- journal article
- Language
- en
- Open access
- closed
- Locations
- Nursing Research (publishedVersion)
- PubMed (publishedVersion)
- Topics
- Social Sciences › Decision Sciences › Statistics, Probability and Uncertainty › Meta-analysis and systematic reviews
- Social Sciences › Business, Management and Accounting › Management Information Systems › Optics and Image Analysis
- Social Sciences › Decision Sciences › Statistics, Probability and Uncertainty › scientometrics and bibliometrics research
- Keywords
- grey literature, meta-analysis, unpublished research, publication bias, effect size inflation, methodological quality, evidence synthesis, moderator analysis
- MeSH terms
- Periodicals as Topic, Publications, Publishing, Research Design, Meta-Analysis as Topic, Journalism, Medical, Information Dissemination
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