title: Influence of trial sample size on treatment effect estimates: meta-epidemiological study.
abstract: OBJECTIVE
  To assess the influence of trial sample size on treatment effect estimates within meta-analyses.
  
  
  METHODS
  Meta-epidemiological study.
  
  
  METHODS
  93 meta-analyses (735 randomised controlled trials) assessing therapeutic interventions with binary outcomes, published in the 10 leading journals of each medical subject category of the Journal Citation Reports or in the Cochrane Database of Systematic Reviews.
  
  
  METHODS
  Sample size, outcome data, and risk of bias extracted from each trial.
  
  
  RESULTS
  Trials within each meta-analysis were sorted by their sample size: using quarters within each meta-analysis (from quarter 1 with 25% of the smallest trials, to quarter 4 with 25% of the largest trials), and using size groups across meta-analyses (ranging from <50 to ≥ 1000 patients). Treatment effects were compared within each meta-analysis between quarters or between size groups by average ratios of odds ratios (where a ratio of odds ratios less than 1 indicates larger effects in smaller trials).
  
  
  RESULTS
  Treatment effect estimates were significantly larger in smaller trials, regardless of sample size. Compared with quarter 4 (which included the largest trials), treatment effects were, on average, 32% larger in trials in quarter 1 (which included the smallest trials; ratio of odds ratios 0.68, 95% confidence interval 0.57 to 0.82), 17% larger in trials in quarter 2 (0.83, 0.75 to 0.91), and 12% larger in trials in quarter 3 (0.88, 0.82 to 0.95). Similar results were obtained when comparing treatment effect estimates between different size groups. Compared with trials of 1000 patients or more, treatment effects were, on average, 48% larger in trials with fewer than 50 patients (0.52, 0.41 to 0.66) and 10% larger in trials with 500-999 patients (0.90, 0.82 to 1.00).
  
  
  CONCLUSIONS
  Treatment effect estimates differed within meta-analyses solely based on trial sample size, with stronger effect estimates seen in small to moderately sized trials than in the largest trials.
authors.affiliation: INSERM, U738, Paris, France. agnes.dechartres@htd.aphp.fr
authors.forename: Agnes
authors.initials: A
authors.lastname: Dechartres
authors.forename: Ludovic
authors.initials: L
authors.lastname: Trinquart
authors.forename: Isabelle
authors.initials: I
authors.lastname: Boutron
authors.forename: Philippe
authors.initials: P
authors.lastname: Ravaud
mesh_terms: D015982:Bias; D006801:Humans; D018579:Patient Selection; D010506:Periodicals as Topic; D016032:Randomized Controlled Trials as Topic / Q000379:methods*