Grey Literature and Preprints
Core summary
Grey literature includes research outputs not published in traditional peer-reviewed journals — theses, conference abstracts, preprints, government reports, and trial registry records. Ignoring grey literature leads to publication bias.
Detailed explanation
Detailed explanation
Grey literature is any scholarly material that is not published through traditional commercial publishing channels. It matters because studies with positive results are more likely to be published in journals (publication bias), while negative or inconclusive studies may only appear in grey literature sources. Key grey literature sources: Preprint servers: MedRxiv (medicine) and BioRxiv (biology) host research manuscripts before peer review. They provide early access to findings — critical during health emergencies like COVID-19. Limitation: These papers have NOT been peer-reviewed and may contain errors. Dissertations and theses: Contain detailed original research, often with more methodological detail than journal articles. Sources include ProQuest Dissertations, university repositories, and national thesis databases. Conference abstracts and proceedings: Many studies are presented at conferences but never published as full papers. Sources include conference databases, society websites, and Embase (which indexes conference abstracts). Trial registries: ClinicalTrials.gov, WHO ICTRP, and ISRCTN contain protocols and sometimes results of clinical trials, including those that were never published in journals. Government and organization reports: WHO reports, CDC publications, Health Technology Assessments (HTA), and national health authority documents. For systematic reviews, including grey literature is recommended by Cochrane and PRISMA to reduce publication bias. However, the quality of grey literature varies widely, so it must be critically appraised just like journal articles.
Clinical example
During the COVID-19 pandemic, preprints on MedRxiv became essential for rapid information sharing. The initial evidence on vaccine efficacy was first available as preprints weeks before peer-reviewed publication. However, some preprints later proved to be flawed, highlighting the need for cautious interpretation.
Research example
Hopewell et al. demonstrated that including grey literature in systematic reviews changed the pooled effect estimate in 19% of meta-analyses compared to using only published studies, confirming that excluding grey literature can bias review conclusions.
Knowledge check
Q1. Why is grey literature important for systematic reviews?
Q2. What is a critical limitation of preprint servers like MedRxiv?
Q3. Including grey literature in systematic reviews has been shown to change the pooled effect estimate in approximately what percentage of meta-analyses?