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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 …