title: Does the inclusion of grey literature influence estimates of intervention effectiveness reported in meta-analyses?
abstract: BACKGROUND
  The inclusion of only a subset of all available evidence in a meta-analysis may introduce biases and threaten its validity; this is particularly likely if the subset of included studies differ from those not included, which may be the case for published and grey literature (unpublished studies, with limited distribution). We set out to examine whether exclusion of grey literature, compared with its inclusion in meta-analysis, provides different estimates of the effectiveness of interventions assessed in randomised trials.
  
  
  METHODS
  From a random sample of 135 meta-analyses, we identified and retrieved 33 publications that included both grey and published primary studies. The 33 publications contributed 41 separate meta-analyses from several disease areas. General characteristics of the meta-analyses and associated studies and outcome data at the trial level were collected. We explored the effects of the inclusion of grey literature on the quantitative results using logistic-regression analyses.
  
  
  RESULTS
  33% of the meta-analyses were found to include some form of grey literature. The grey literature, when included, accounts for between 4.5% and 75% of the studies in a meta-analysis. On average, published work, compared with grey literature, yielded significantly larger estimates of the intervention effect by 15% (ratio of odds ratios=1.15 [95% CI 1.04-1.28]). Excluding abstracts from the analysis further compounded the exaggeration (1.33 [1.10-1.60]).
  
  
  CONCLUSIONS
  The exclusion of grey literature from meta-analyses can lead to exaggerated estimates of intervention effectiveness. In general, meta-analysts should attempt to identify, retrieve, and include all reports, grey and published, that meet predefined inclusion criteria.
authors.affiliation: Thomas C Chalmers Center for Systematic Reviews, Children's Hospital of Eastern Ontario Research Institute, Ottawa, Canada.
authors.forename: L
authors.initials: L
authors.lastname: McAuley
authors.forename: B
authors.initials: B
authors.lastname: Pham
authors.forename: P
authors.initials: P
authors.lastname: Tugwell
authors.forename: D
authors.initials: D
authors.lastname: Moher
mesh_terms: D002986:Clinical Trials as Topic / Q000592:standards; D004358:Drug Therapy / Q000592:standards; D016239:MEDLINE; D015201:Meta-Analysis as Topic; D010506:Periodicals as Topic; D011642:Publications / Q000706:statistics & numerical data