The inclusion of grey literature in meta-analyses and reviews is controversial. We examine both the advantages and challenges of including grey literature in meta-analyses. An exemplar meta-analysis of behavioral parenting interventions on parent behavior, child behavior, and parent adjustment outcomes is used to demonstrate these issues. It also explores the influence of the inclusion of grey literature on outcomes, including whether effect sizes are affected by the inclusion of grey literature and describes the challenges of searching for grey literature using traditional search engines such as Google and Yahoo. Homogeneity and publication bias are also examined. Based on these results, recommendations are presented for meta-analysts and researchers.
