title: Multiplicity of data in trial reports and the reliability of meta-analyses: empirical study.
abstract: OBJECTIVE
  To examine the extent of multiplicity of data in trial reports and to assess the impact of multiplicity on meta-analysis results.
  
  
  METHODS
  Empirical study on a cohort of Cochrane systematic reviews.
  
  
  METHODS
  All Cochrane systematic reviews published from issue 3 in 2006 to issue 2 in 2007 that presented a result as a standardised mean difference (SMD). We retrieved trial reports contributing to the first SMD result in each review, and downloaded review protocols. We used these SMDs to identify a specific outcome for each meta-analysis from its protocol. Review methods Reviews were eligible if SMD results were based on two to ten randomised trials and if protocols described the outcome. We excluded reviews if they only presented results of subgroup analyses. Based on review protocols and index outcomes, two observers independently extracted the data necessary to calculate SMDs from the original trial reports for any intervention group, time point, or outcome measure compatible with the protocol. From the extracted data, we used Monte Carlo simulations to calculate all possible SMDs for every meta-analysis.
  
  
  RESULTS
  We identified 19 eligible meta-analyses (including 83 trials). Published review protocols often lacked information about which data to choose. Twenty-four (29%) trials reported data for multiple intervention groups, 30 (36%) reported data for multiple time points, and 29 (35%) reported the index outcome measured on multiple scales. In 18 meta-analyses, we found multiplicity of data in at least one trial report; the median difference between the smallest and largest SMD results within a meta-analysis was 0.40 standard deviation units (range 0.04 to 0.91).
  
  
  CONCLUSIONS
  Multiplicity of data can affect the findings of systematic reviews and meta-analyses. To reduce the risk of bias, reviews and meta-analyses should comply with prespecified protocols that clearly identify time points, intervention groups, and scales of interest.
authors: Nordic Cochrane Centre, Rigshospitalet, Blegdamsvej 9, DK-2100 Copenhagen Ø, Denmark. bt@cochrane.dk
authors: Britta
authors: B
authors: Tendal
authors: Eveline
authors: E
authors: Nüesch
authors: Julian P T
authors: JP
authors: Higgins
authors: Peter
authors: P
authors: Jüni
authors: Peter C
authors: PC
authors: Gøtzsche
mesh_terms: D015982:Bias; D006801:Humans; D015201:Meta-Analysis as Topic; D009010:Monte Carlo Method; D016032:Randomized Controlled Trials as Topic; D012196:Review Literature as Topic