In randomized control trials (RCTs), a focus on average differences between treatment arms often limits our understanding of whether individuals show clinically significant improvement or deterioration. The present study examined differences in individual-level clinical significance trajectories between Concurrent Treatment of PTSD and Substance Use Disorders Using Prolonged Exposure (COPE) and Relapse Prevention (RP). Eighty-one treatment-seeking veterans with a comorbid PTSD/SUD diagnosis were randomized to COPE or RP; data from an additional n = 48 patients who did not meet criteria for both disorders was used to establish a normative threshold. A newly developed, modernized approach to the Jacobson and Truax (1991) clinically significant change framework, using (a) moderated nonlinear factor analysis (MNLFA) scale scoring and (b) measurement error-corrected multilevel modeling (MEC-MLM) was used; this approach was compared to other approaches using conventional total scores and/or assuming no measurement error. Using a conventional approach to estimating the Reliable Change Index (RCI) yielded no differences between COPE and RP in the percentage of patients achieving statistically significant improvement (SSI; 88.9% for both groups). However, under MNLFA/MEC-MLM, higher percentages of patients receiving COPE (75.0%) achieved SSI compared to RP (40.7%). Findings suggest that, even though COPE and RP appear to reduce the same number of PTSD symptoms, MNLFA scoring of outcome measures gives greater weight to interventions that target and reduce "hallmark" PTSD symptoms.
