title: The effects of excluding patients from the analysis in randomised controlled trials: meta-epidemiological study.
abstract: OBJECTIVE
  To examine whether excluding patients from the analysis of randomised trials are associated with biased estimates of treatment effects and higher heterogeneity between trials.
  
  
  METHODS
  Meta-epidemiological study based on a collection of meta-analyses of randomised trials.
  
  
  METHODS
  14 meta-analyses including 167 trials that compared therapeutic interventions with placebo or non-intervention control in patients with osteoarthritis of the hip or knee and used patient reported pain as an outcome.
  
  
  METHODS
  Effect sizes were calculated from differences in means of pain intensity between groups at the end of follow-up, divided by the pooled standard deviation. Trials were combined by using random effects meta-analysis. Estimates of treatment effects were compared between trials with and trials without exclusions from the analysis, and the impact of restricting meta-analyses to trials without exclusions was assessed.
  
  
  RESULTS
  39 trials (23%) had included all patients in the analysis. In 128 trials (77%) some patients were excluded from the analysis. Effect sizes from trials with exclusions tended to be more beneficial than those from trials without exclusions (difference -0.13, 95% confidence interval -0.29 to 0.04). However, estimates of bias between individual meta-analyses varied considerably (tau(2)=0.07). Tests of interaction between exclusions from the analysis and estimates of treatment effects were positive in five meta-analyses. Stratified analyses indicated that differences in effect sizes between trials with and trials without exclusions were more pronounced in meta-analyses with high between trial heterogeneity, in meta-analyses with large estimated treatment benefits, and in meta-analyses of complementary medicine. Restriction of meta-analyses to trials without exclusions resulted in smaller estimated treatment benefits, larger P values, and considerable decreases in between trial heterogeneity.
  
  
  CONCLUSIONS
  Excluding patients from the analysis in randomised trials often results in biased estimates of treatment effects, but the extent and direction of bias is unpredictable. Results from intention to treat analyses should always be described in reports of randomised trials. In systematic reviews, the influence of exclusions from the analysis on estimated treatment effects should routinely be assessed.
authors.affiliation: Institute of Social and Preventive Medicine, University of Bern, Switzerland.
authors.forename: Eveline
authors.initials: E
authors.lastname: Nüesch
authors.forename: Sven
authors.initials: S
authors.lastname: Trelle
authors.forename: Stephan
authors.initials: S
authors.lastname: Reichenbach
authors.forename: Anne W S
authors.initials: AW
authors.lastname: Rutjes
authors.forename: Elizabeth
authors.initials: E
authors.lastname: Bürgi
authors.forename: Martin
authors.initials: M
authors.lastname: Scherer
authors.forename: Douglas G
authors.initials: DG
authors.lastname: Altman
authors.forename: Peter
authors.initials: P
authors.lastname: Jüni
mesh_terms: D015982:Bias; D003625:Data Collection / Q000379:methods*; D006801:Humans; D015201:Meta-Analysis as Topic; D015207:Osteoarthritis, Hip / Q000628:therapy; D020370:Osteoarthritis, Knee / Q000628:therapy; D010146:Pain / Q000517:prevention & control; D016032:Randomized Controlled Trials as Topic / Q000379:methods* / Q000706:statistics & numerical data; D018401:Sample Size