Sample size determination for studies designed to estimate covariate‐dependent reference quantile curves

Accuracy and sample size issues concerning the estimation of covariate‐dependent quantile curves are considered. It is proposed to measure the precision of an estimate of the p th quantile at a given covariate value by the probability with which this estimate lies between the p 1 th and p 2 th quantile, where p 1  <  p  <  p 2 . Requiring that this probability exceeds a given confidence bound for all covariate values in a specified range leads to a sample size criterion. Approximate formulae for the precision and sample size are derived for the normal parametric regression approach and for the semiparametric quantile regression method. A simulation study is performed to evaluate the accuracy of the approximations. Numerical evaluations show that rather large numbers of subjects are needed to construct quantile curves with a reasonable amount of accuracy, especially if the quantile regression method is applied. Copyright © 2013 John Wiley & Sons, Ltd.