Sample size determination for the confidence interval of mean comparison adjusted by multiple covariates

The current method of determining sample size for confidence intervals does not accommodate multiple covariate adjustment. Under the normality assumption, the effect of multiple covariate adjustment on the standard error of the mean comparison is related to a Hotelling T <Superscript>2</Superscript> statistic. Sample size can be calculated to obtain a desired probability of achieving a predetermined width in the confidence interval of the mean comparison with multiple covariate adjustment, given that the confidence interval includes the population parameter. Copyright Springer-Verlag Berlin Heidelberg 2013