An Artifact in Pretest-Posttest Designs

Abstract

A marked decrease in arrest rates-up to 70%-was noted in the evaluation of a juve nile corrections program. This so-called "suppression effect" has been widely publicized as proof of the effectiveness of correctional intervention. In this article we show that the sup pression effect can easily be explained as a statistical artifact, regression to the mean, assuming that judges base their sentencing decisions on juveniles' prior records. We con clude that before/after comparisons should not be used to evaluate delinquency programs.