Processes of Change Experienced in Psychotherapy Sessions: Development and Validation of the PrOCESs-Inventory

Understanding therapeutic change and informing psychotherapy practice requires instruments to assess in-session change processes. This study aimed to develop a practice-oriented instrument assessing processes of change experienced within psychotherapy sessions, grounded in patient and therapist accounts of how therapeutic conversations contribute to change. Item generation followed a mixed-method, bottom-up, multi-informant approach based on qualitative interviews with patients and therapists. Following cognitive pretesting and iterative item reduction, data from 418 participants were analyzed in separate construction and validation subsamples generated using the Solomon method. Exploratory and confirmatory factor analyses were conducted, and convergent validity was examined using correlations with the Session Questionnaire for General and Differential Individual Psychotherapy (STEP) and the Individual Therapy Process Questionnaire (ITPQ). Analyses supported a six-factor structure comprising 24 items: Relationship, Perceiving and Expressing Feelings and Thoughts, Becoming Conscious of Biographical Experiences, New Perspective on Problems, Self-Disclosure, and Receiving Advice. The model showed good fit and good to excellent composite reliability. Correlations supported convergent validity. The PrOCESs-Inventory is a reliable, empirically grounded instrument for assessing processes of change experienced within psychotherapy sessions. It may support psychotherapy process research, clinical supervision, and feedback-informed treatment.