Incremental validity of the Rorschach Prognostic Rating Scale over the MMPI Ego Strength Scale and IQ.

A recent meta-analysis found that the Rorschach Prognostic Rating Scale (RPRS) had a strong ability to predict subsequent outcome (r = .44, N = 783; Meyer & Handler, 1997, this issue). However, that review did not directly address questions of incremental validity. This article focuses on the ability of the RPRS to predict outcome after taking into account other sources of data. Across studies that examined both the RPRS and the MMPI Ego Strength scale, the RPRS had a strong ability to predict outcome (r = .40, N = 187), whereas the MMPI scale did not (r = .02, N = 280). Nine studies examined the RPRS along with an intelligence test and allowed direct numerical estimates of incremental validity to be calculated. Across studies, the RPRS demonstrated strong incremental validity after controlling for intelligence (incremental r = .36, N = 358). It is clear that the Rorschach can make unique contributions to understanding clinically relevant processes in ways that self-reports or measured intelligence cannot. Contemporary Rorschach scales should continue to be evaluated for their distinctive and incremental contribution to clinical practice.