The phase model of psychotherapy outcome: Domain-specific trajectories of change in outpatient treatment

Abstract

Objective: The Phase model of psychotherapy outcome assumes a log-linear trajectory of change and allows predictions about the three outcome domains of Well-being, Symptoms, and Interpersonal functioning. Although the model has been partly validated, little is known about the types of trajectories in the specific domains and about the transferability of the model to treatments of longer duration. This study tested whether the domain-specific change trajectories followed a log-linear curve as postulated by the Phase model. Furthermore, it was examined whether the speed of change differed between the domains. Method: Growth curves were modeled using hierarchical linear modeling on an outpatient sample (N = 351), with treatment duration averaging 52 sessions. Results: A log-linear curve best explained the change trajectories of the domains Well-being and Symptoms as well as the Global score of psychopathology, whereas Interpersonal functioning tends to improve in a linear pattern. Estimated slopes were biggest for Well-being and smallest for Interpersonal functioning. Conclusions: In the present study, the predictions of the Phase model concerning multidimensional phases were validated. The finding that trajectories of change are not uniformly log-linear, but either log-linear or linear in a domain-specific manner, is partly inconsistent with the model.