Regression toward the mean associated with measurement error and the identification of improvement and deterioration in psychotherapy.

Several factors can cause posttherapy scores to regress (relative to pretherapy scores) toward a group mean. This article focuses on regression effects that are due to measurement error, and on methods used to evaluate pre- to posttherapy score changes that attempt to take these regression effects into account. Statistical and practical problems associated with these methods are discussed. In particular, a statistical problem is identified in the most recent article that advocates use of these methods. Consequences of ignoring this problem and a way of correcting it are presented. Five practical problems encountered in using a correct classical true-score theory method of identifying reliable changes (based on derivations of H. Gulliksen [1950] and F. M. Lord and M. R. Novick [1968]) are then presented. Finally, objectives of these methods are distinguished from objectives of other strategies used to evaluate changes in psychotherapy.