Objective: The dose-response model of change in psychotherapy posits that each session of therapy is incrementally beneficial across patients. The contrasting good-enough level model suggests that patients improve at different rates in therapy and discontinue treatment when they are satisfied with their improvement. Support for each theory has been mixed, and many prior studies have relied on samples of patients receiving unstructured treatment approaches. We conducted this study to compare these two theories across two manualized treatments for posttraumatic stress disorder (PTSD).
Method: Two hundred eighty-four female veterans and military service members with PTSD (Mage = 44.79; 54.6% White non-Hispanic, 6.7% Black non-Hispanic, 37% other) were randomized to receive 10 sessions of prolonged exposure (PE), a trauma-focused therapy, or present-centered therapy (PCT), a non-trauma-focused therapy. Participants completed the PTSD Checklist (PCL) at even-numbered treatment sessions, and the timing of dropout/treatment completion was monitored.
Results: The point of highest risk for dropout differed between the treatments, with risk in PE corresponding to the beginning of imaginal exposures. In the PE condition, but not in PCT, a higher number of sessions completed increased the likelihood of achieving reliable clinically significant improvement. Across treatments, the rate of change in PTSD symptoms did not differ according to the number of sessions completed (b = 0.06, p = .687).
Conclusions: Findings support the dose-response model of change in psychotherapy. There were notable differences in dropout across the treatment conditions, including rates, timing, and implications for outcomes. These differences likely reflect differences in content between the protocols. (PsycInfo Database Record (c) 2023 APA, all rights reserved).