The method proposed by Sheiner and Beal plays a cornerstone role in the evaluation of the predictive performance. Recently, the Sheiner and Beal's method has been intensively used in pharmacokinetic studies, however, this method has been used in a somewhat confusing way by several authors. Therefore, if one wants to use this method, one might find different examples to follow in the pharmacokinetic literature. Meanwhile, several detailed points were not given by Sheiner and Beal in their original paper. Clearly, the principles suggested by Sheiner and Beal are simple, but the practical application is something of an art. In this work we addressed these detailed points and indicated the confusion in using the Sheiner and Beal's method.