[Assessing common factors in psychotherapy: psychometric properties of a new time-economic instrument (WIFA-k)].

Research on common and differential factors in the therapeutic process is impeded by the lack of instruments suitable for assessing common change mechanisms. This study presents the psychometric properties of a newly developed time-economic instrument (WIFA-k), which was designed to assess common factors of psychotherapy as designed by Grawe. Within a multi-center study comparing the efficacy of cognitive therapy and psychodynamic therapy in the treatment of social phobia, 6 raters assessed 25 randomly selected, videotaped therapy sessions of each treatment approach, and evaluated common factors using the Wifa-k. Interrater-reliability was found to be high for the items "resource activation", "motivational clarification" and "mastery" and low for the items "therapeutic relationship" and "problem activation". Ways to increase reliability and validity of the scale are discussed.