Simply put, grey literature encompasses a body of literature not indexed in standard bibliographic databases. Grey Literature Report defines grey literature as “non-conventional, fugitive, and sometimes ephemeral publications.”1 Major grey literature categories include, but are not limited to2:
The inclusion of grey literature in any systematic search of the literature is essential to help minimize publication bias.
Balshem H, Stevens A, Ansari M, et al. Finding Grey Literature Evidence and Assessing for Outcome and Analysis Reporting Biases When Comparing Medical Interventions: AHRQ and the Effective Health Care Program. Rockville, MD: Agency for Healthcare Research and Quality; November 2013.
Lefebvre C, Glanville J, Briscoe S, et al. Searching for and selecting studies. In: Higgins J, Thomas J, Chandler J, et al., eds. Cochrane Handbook for Systematic Reviews of Interventions. version 6.2: Cochrane; 2021.
JBI Manual for Evidence Synthesis. In: Aromataris E, Munn Z, eds.: JBI; 2020: https://synthesismanual.jbi.global/. Accessed April 21, 2021.
Item 9: "Describe all intended information sources (such as electronic databases, contact with study authors, trial registers or other grey literature sources) with planned dates of coverage."
Shamseer L, Moher D, Clarke M, et al. Preferred reporting items for systematic review and meta-analysis protocols (PRISMA-P) 2015: elaboration and explanation. BMJ (Clinical research ed). 2015;350:g7647.
1. What is Grey Literature? The New York Academy of Medicine. Grey Literature Report Web site. https://www.greylit.org/about. Accessed April 21, 2021.
2. Alberani V, De Castro Pietrangeli P, Mazza AM. The use of grey literature in health sciences: a preliminary survey. Bulletin of the Medical Library Association. 1990;78(4):358-363.