The impact of trial baseline imbalances should be considered in systematic reviews: a methodological case study.
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.
Trowman
Rebecca
R
Epidemiology and Biostatistics, University of Leeds, Leeds LS2 9LN, UK. rebecca_trowman@hotmail.com
Dumville
Jo C
JC
Torgerson
David J
DJ
Cranny
Gillian
G
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
D002118:Calcium