Supplementary Material for: Which Mechanism Kicks in When? Temporal Changes in the Effect of Transtheoretical Factors on Symptom Distress Over the Course of Therapy

Introduction: The mechanisms of change in psychological therapies have not yet been sufficiently empirically validated. In particular, the time course of their effects on patients’ symptom distress is unknown. Therefore, the aims of the present study were (1) to examine the effects of four patient experiences, namely interpersonal (IE), coping (CE), affective (AE), and resource-related experiences (RE), on symptom distress within and between patients, (2) to consider the temporal development of the within-patient effects, and (3) to examine their associations with the final treatment outcome. Methods: Within- and between-patient effects of IE, CE, AE, and RE on session-level symptom distress were calculated for n = 2208 outpatients treated by n = 253 therapists. Patient experiences were measured after each session using session reports, outcome was assessed with the HSCL-11 at the beginning of each session and with the OQ-30 after treatment. Linear mixed models (LMMs) were used to examine the within- and between-patient associations. Temporal changes in the within-patient effects were modelled using rolling windows and three-level LMMs (sessions nested in windows and patients). Results: Within-patient components of all four patient experiences were significantly associated with symptom distress (b = –0.006 to b = –0.057), while none of the between-patient components were. The effects of all four patient experiences diminished over time (b = 0.000 to b = 0.001). The effects of IE, CE, and RE remained negative and significant, but the effect of AE crossed zero and became significantly positive with an increasing window number. A greater decrease in the within-patient effects of IE (b = –0.001) and CE (b = –0.001) on HSCL-11 was associated with better final treatment outcomes. Conclusion: The results suggest that all four change factors are drivers of successful therapy, particularly in the early sessions. Over the course of treatment their impact on symptom distress diminishes, which appears to be beneficial for the final treatment outcome. In the future, differences between patients in the temporal dynamics of change factors should be investigated to inform transtheoretical and process-based treatments that support therapists in personalizing their treatments to individual patients.