Grey Literature for Health Sciences and Medicine

When conducting a systematic review or scoping review, it is important to include grey literature in your search strategy to ensure that you are capturing all relevant information. 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
 

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
 

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

The Cochrane Handbook for Systematic Reviews of Interventions recommends searching for grey literature as a means of reducing publication bias and ensuring that all relevant studies are included in the review.

See:

• Cochrane Handbook  - Technical Supplement to Chapter 4: Searching for, and selecting studies.
• Cochrane Handbook - Other sources, part 2, chapter 4.3.5:
• Cochrane Handbook - Documenting and reporting the search process, Chapter 4.5

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and other types of evidence syntheses, and emphasises the importance of searching for grey literature to identify unpublished and hard-to-find studies.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, Munn Z (Editors). JBI Manual for Evidence Synthesis. JBI, 2020.
  See search strategy notes for each Systematic Review Type and Scoping Reviews

The Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA) guidelines provide a framework for reporting systematic reviews and include a checklist item that calls for reporting the search strategy used for grey literature.

See:

PRISMA 2020 Checklist

PRISMA-S: PRISMA Search Reporting Extension

PRISMA 2020 flow diagram for new systematic reviews which included searches of databases, registers and other sources

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