When conducting a systematic review or scoping review, it is important to consider grey literature. The Cochrane, JBI, and PRISMA guidelines highly recommend or mandate searching for grey literature as part of the review process.
Adams, J., Hillier-Brown, F. C., Moore, H. J., Lake, A. A., Araujo-Soares, V., White, M., & Summerbell, C. (2016). Searching and synthesising ‘grey literature’and ‘grey information’ in public health: critical reflections on three case studies. Systematic reviews, 5, 1-11. Full Text
Godin, K., Stapleton, J., Kirkpatrick, S. I., Hanning, R. M., & Leatherdale, S. T. (2015). Applying systematic review search methods to the grey literature: a case study examining guidelines for school-based breakfast programs in Canada. Systematic Reviews, 4(1), 1-10. Full Text
Hunter, K. E., Webster, A. C., Page, M. J., Willson, M., McDonald, S., Berber, S., ... & Seidler, A. L. (2022). Searching clinical trials registers: guide for systematic reviewers. BMJ, 377: e068791. Full Text
Mahood, Q., Van Eerd, D., & Irvin, E. (2014). Searching for grey literature for systematic reviews: challenges and benefits. Research synthesis methods, 5(3), 221-234. Full Text
Paez, A. (2017). Gray literature: An important resource in systematic reviews. Journal of Evidence‐Based Medicine, 10(3), 233-240. Full Text
It is highly desirable, for authors of Cochrane Reviews of interventions, to search relevant grey literature sources such as reports, dissertations, theses, and conference abstracts.
See:
The Joanna Briggs Institute (JBI) Reviewer's Manual provides guidance on conducting systematic reviews and other types of evidence syntheses, and emphasises the importance of searching for grey literature to identify unpublished and hard-to-find studies.
See: Aromataris E, Lockwood C, Porritt K, Pilla B, Jordan Z, editors. JBI Manual for Evidence Synthesis. JBI; 2024. Available from: https://synthesismanual.jbi.global.
2.4.6.1 Grey literature
5.2.4.3 Searching for grey literature
10.3.7.1 (Scoping Reviews) Search strategy
The PRISMA (Preferred Reporting Items for Systematic reviews and Meta-Analyses) reporting guidelines are designed to improve the reporting of systematic reviews.
Item #6 - Specify all databases, registers, websites, organisations, reference lists and other sources searched or consulted to identify studies. Specify the date when each source was last searched or consulted.
Item #7 - Present the full search strategies for all databases, registers and websites, including any filters and limits used.
See expanded information on Items #6 & #7
For systematic reviews which include searches of databases, registers and other sources
Statement / Explanatory paper:
Rethlefsen, M. L., Kirtley, S., Waffenschmidt, S., Ayala, A. P., Moher, D., Page, M. J., & Koffel, J. B. (2021). PRISMA-S: an extension to the PRISMA statement for reporting literature searches in systematic reviews. Systematic reviews, 10, 1-19. Full Text
Campbell reviews should also include both formally published and unpublished research reports (referred to as the grey literature) such as dissertations, technical reports, and conference presentations.
...all studies meeting the eligibility criteria should be included, whether or not they have been formally published. Thus, dissertations, technical reports, conference papers, and other such grey literature should be included along with studies more formally published in journals and books.
The Campbell Collaboration. Campbell systematic reviews: policies and guidelines, Campbell Policies and Guidelines (p. 9, 30).
DOI: 10.4073/cpg.2016.1