The impact of trial baseline imbalances should be considered in systematic reviews: a methodological case study

Rebecca Trowman, Jo C. Dumville, David J. Torgerson, Gillian Cranny

Research output: Contribution to journalLiterature reviewpeer-review


Objectives: 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.

Study Design and Setting: 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.

Conclusion: The meta-analyst should always consider the possibility of baseline imbalances and adjustments should be made wherever possible. (C) 2007 Elsevier Inc. All rights reserved.

Original languageEnglish
Pages (from-to)1229-1233
Number of pages5
JournalJournal of Clinical Epidemiology
Issue number12
Publication statusPublished - Dec 2007


  • meta-analysis
  • baseline imbalances
  • baseline differences
  • systematic reviews
  • clinical trials
  • randomization

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