The reliability of self-image change scores in psychotherapy research: An application of generalizability theory.

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Abstract
The aim of this study was to estimate the reliability of the pre- to posttreatment change scores for 3 different self-image aspects, Attack, Love, and Control. To measure self-image, we used the Norwegian version of the introject surface of Benjamin's (1974) structural analysis of social behavior. The article introduces Generalizability (G-) theory, combined with the recent concept of tolerance for error, as a framework for estimating the reliability and precision of change scores in 1- and 2-facet designs. Data were obtained from the Norwegian Multi-Site Study of Process and Outcome in Psychotherapy, including 291 outpatients. The mean number of treatment sessions was 47. The results show that change scores may be highly reliable. Generalizability coefficients resting on the relative and absolute score interpretations, respectively, for both the Love and Attack change scores reached acceptable levels. The reliability of the Control change score was, however, poor. G-theory combined with the error-tolerance concept proved to be a helpful framework for assessing the dependability of change scores in a psychotherapy research setting. (PsycINFO Database Record (c) 2010 APA, all rights reserved).