• Adrián V Hernández, Ewout W Steyerberg, and J Dik F Habbema.
• Center for Clinical Decision Sciences, Department of Public Health, Erasmus Medical Center, P.O. Box 1738, Rotterdam 3000 DR, The Netherlands. a.hernandez@erasmusmc.nl
• J Clin Epidemiol. 2004 May 1; 57 (5): 454-60.

ObjectiveRandomized controlled trials (RCTs) with dichotomous outcomes may be analyzed with or without adjustment for baseline characteristics (covariates). We studied type I error, power, and potential reduction in sample size with several covariate adjustment strategies.Study Design And SettingLogistic regression analysis was applied to simulated data sets (n=360) with different treatment effects, covariate effects, outcome incidences, and covariate prevalences. Treatment effects were estimated with or without adjustment for a single dichotomous covariate. Strategies included always adjusting for the covariate ("prespecified"), or only when the covariate was predictive or imbalanced.ResultsWe found that the type I error was generally at the nominal level. The power was highest with prespecified adjustment. The potential reduction in sample size was higher with stronger covariate effects (from 3 to 46%, at 50% outcome incidence and covariate prevalence) and independent of the treatment effect. At lower outcome incidences and/or covariate prevalences, the reduction was lower.ConclusionWe conclude that adjustment for a predictive baseline characteristic may lead to a potentially important increase in power of analyses of treatment effect. Adjusted analysis should, hence, be considered more often for RCTs with dichotomous outcomes.

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