Control or overcontrol for covariates?

Covariate adjustment can adjust for baseline differences in randomised controlled trials (RCTs) that may have arisen by chance. Furthermore, even if the groups do not differ significantly on any factors, using baseline variables that may be related to the outcome as covariates can reduce the within-group variance, thus increasing the accuracy of the estimates of treatment effects and the power of the statistical test. However, improper use of covariate adjustment can either magnify or diminish the difference between the groups. In RCTs, covariates must be chosen carefully and should not include variables that may have been affected by the treatment itself. The use of covariate adjustment in cohort studies is even more fraught and may result in paradoxical situations, in which there can be opposite interpretations of the results. Published by the BMJ Publishing Group Limited. For permission to use (where not already granted under a licence) please go to http://www.bmj.com/company/products-services/rights-and-licensing/