title: The impact of trial baseline imbalances should be considered in systematic reviews: a methodological case study.
abstract: OBJECTIVE
  It is possible for baseline imbalances to occur between treatment groups for one or more variables in a randomized controlled trial, although the identification and detection of baseline imbalances remain controversial. If trials with baseline imbalances are combined in a meta-analysis, then this may result in misleading conclusions.
  
  
  METHODS
  The identification and consequences of baseline imbalances in meta-analyses are discussed. Metaregression using mean baseline scores as a covariate is proposed as a potential method for adjusting baseline imbalances within meta-analysis. We will use a recent systematic review looking at the effect of calcium supplements on weight as an illustrative case study.
  
  
  RESULTS
  Meta-analysis conducted using the mean final values of the treatment groups as the outcome resulted in an apparent, statistically significant, treatment effect. However, using a meta-analysis of baseline values, this was shown to be due to the baseline imbalance between treatment groups, rather than as a result of any intervention received by the participants. Applying the method of metaregression demonstrated that there was in fact a smaller, statistically insignificant effect between treatment groups.
  
  
  CONCLUSIONS
  The meta-analyst should always consider the possibility of baseline imbalances and adjustments should be made wherever possible.
authors.affiliation: Epidemiology and Biostatistics, University of Leeds, Leeds LS2 9LN, UK. rebecca_trowman@hotmail.com
authors.forename: Rebecca
authors.initials: R
authors.lastname: Trowman
authors.forename: Jo C
authors.initials: JC
authors.lastname: Dumville
authors.forename: David J
authors.initials: DJ
authors.lastname: Torgerson
authors.forename: Gillian
authors.initials: G
authors.lastname: Cranny
mesh_terms: D015982:Bias; D002118:Calcium / Q000494:pharmacology; D019587:Dietary Supplements; D006801:Humans; D015201:Meta-Analysis as Topic; D016032:Randomized Controlled Trials as Topic; D012196:Review Literature as Topic; D016896:Treatment Outcome; D015431:Weight Loss / Q000187:drug effects
chemical_list: D002118:Calcium