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.
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.
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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and other types of evidence syntheses, and emphasises the importance of searching for grey literature to identify unpublished and hard-to-find studies.
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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.
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Campbell Collaboration on Grey Literature (Section 3.3 page 17)
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