title: Publication bias in meta-analyses from the Cochrane Database of Systematic Reviews.
abstract: We used a Bayesian hierarchical selection model to study publication bias in 1106 meta-analyses from the Cochrane Database of Systematic Reviews comparing treatment with either placebo or no treatment. For meta-analyses of efficacy, we estimated the ratio of the probability of including statistically significant outcomes favoring treatment to the probability of including other outcomes. For meta-analyses of safety, we estimated the ratio of the probability of including results showing no evidence of adverse effects to the probability of including results demonstrating the presence of adverse effects.
  
  
  RESULTS
  In the meta-analyses of efficacy, outcomes favoring treatment had on average a 27% (95% Credible Interval (CI): 18% to 36%) higher probability to be included than other outcomes. In the meta-analyses of safety, results showing no evidence of adverse effects were on average 78% (95% CI: 51% to 113%) more likely to be included than results demonstrating that adverse effects existed. In general, the amount of over-representation of findings favorable to treatment was larger in meta-analyses including older studies.
  
  
  CONCLUSIONS
  In the largest study on publication bias in meta-analyses to date, we found evidence of publication bias in Cochrane systematic reviews. In general, publication bias is smaller in meta-analyses of more recent studies, indicating their better reliability and supporting the effectiveness of the measures used to reduce publication bias in clinical trials. Our results indicate the need to apply currently underutilized meta-analysis tools handling publication bias based on the statistical significance, especially when studies included in a meta-analysis are not recent.
authors.affiliation: Faculty of Science, Hasselt University, Diepenbeek, Belgium.
authors.forename: Michal
authors.initials: M
authors.lastname: Kicinski
authors.affiliation: Centre for Primary Care, National Institute for Health Research School for Primary Care Research, Institute of Population Health, University of Manchester, Manchester, U.K.|Centre for Biostatistics, Institute of Population Health, University of Manchester, Manchester, U.K.
authors.forename: David A
authors.initials: DA
authors.lastname: Springate
authors.affiliation: Centre for Primary Care, National Institute for Health Research School for Primary Care Research, Institute of Population Health, University of Manchester, Manchester, U.K.|Centre for Health Informatics, Institute of Population Health, University of Manchester, Manchester, U.K.
authors.forename: Evangelos
authors.initials: E
authors.lastname: Kontopantelis
mesh_terms: D001499:Bayes Theorem; D002986:Clinical Trials as Topic; D016208:Databases, Factual; D015201:Meta-Analysis as Topic; D015990:Placebo Effect; D017594:Publication Bias; D012196:Review Literature as Topic
keywords: Bayesian methods; Cochrane Library; meta-analysis; publication bias; selection model; systematic reviews