Covariate adjustment in randomized controlled trials with dichotomous outcomes increases statistical power and reduces sample size requirements
Journal of Clinical Epidemiology , Volume 57 - Issue 5 p. 454- 460
Objective Randomized 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 Setting Logistic 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. Results We 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. Conclusion We 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.
|Covariate adjustment, Logistic regression, Randomized controlled trials, Sample size, Statistical power, Type I error|
|Journal of Clinical Epidemiology|
|Organisation||Erasmus MC: University Medical Center Rotterdam|
Hernández, A.V, Steyerberg, E.W, & Habbema, J.D.F. (2004). Covariate adjustment in randomized controlled trials with dichotomous outcomes increases statistical power and reduces sample size requirements. Journal of Clinical Epidemiology, 57(5), 454–460. doi:10.1016/j.jclinepi.2003.09.014