title: Impact of allocation concealment on conclusions drawn from meta-analyses of randomized trials.
abstract: BACKGROUND
  Randomized trials without reported adequate allocation concealment have been shown to overestimate the benefit of experimental interventions. We investigated the robustness of conclusions drawn from meta-analyses to exclusion of such trials.
  
  
  METHODS
  Random sample of 38 reviews from The Cochrane Library 2003, issue 2 and 32 other reviews from PubMed accessed in 2002. Eligible reviews presented a binary effect estimate from a meta-analysis of randomized controlled trials as the first statistically significant result that supported a conclusion in favour of one of the interventions.
  
  
  METHODS
  We assessed the methods sections of the trials in each included meta-analysis for adequacy of allocation concealment. We replicated each meta-analysis using the authors' methods but included only trials that had adequate allocation concealment. Conclusions were defined as not supported if our result was not statistically significant.
  
  
  RESULTS
  Thirty-four of the 70 meta-analyses contained a mixture of trials with unclear or inadequate concealment as well as trials with adequate allocation concealment. Four meta-analyses only contained trials with adequate concealment, and 32, only trials with unclear or inadequate concealment. When only trials with adequate concealment were included, 48 of 70 conclusions (69%; 95% confidence interval: 56-79%) lost support. The loss of support mainly reflected loss of power (the total number of patients was reduced by 49%) but also a shift in the point estimate towards a less beneficial effect.
  
  
  CONCLUSIONS
  Two-thirds of conclusions in favour of one of the interventions were no longer supported if only trials with adequate allocation concealment were included.
authors.affiliation: The Nordic Cochrane Centre, Rigshospitalet, DK. jp@cochrance.dk
authors.forename: J
authors.initials: J
authors.lastname: Pildal
authors.forename: A
authors.initials: A
authors.lastname: Hróbjartsson
authors.forename: K J
authors.initials: KJ
authors.lastname: Jørgensen
authors.forename: J
authors.initials: J
authors.lastname: Hilden
authors.forename: D G
authors.initials: DG
authors.lastname: Altman
authors.forename: P C
authors.initials: PC
authors.lastname: Gøtzsche
mesh_terms: D015982:Bias; D003627:Data Interpretation, Statistical; D004311:Double-Blind Method; D006801:Humans; D015201:Meta-Analysis as Topic; D016032:Randomized Controlled Trials as Topic / Q000379:methods*; D012680:Sensitivity and Specificity; D016037:Single-Blind Method; D016896:Treatment Outcome