Statistical analyses
IUT
DER
Statistical analyses were performed with SPSS (Version 27, SPSS Inc., Chicago, IL). Proportions, means, and standard deviations (SDs) were calculated for feasibility measures. Differences in session attendance between the IUT and MCT groups were investigated with an independent t-test and differences in dropout with a Fisher’s exact test. A descriptive analysis investigating the normality of continuous outcome variables was performed. In the preliminary evaluation of treatment effects, multilevel modeling was used to estimate the effects of time and of time by group on continuous outcome measures from the pre-treatment to the post-treatment assessment and from the post-treatment to the follow-up assessment. The maximum likelihood method was used to estimate model parameters. A first-order autoregressive structure with homogenous variances provided the best fit and was thus used as the covariance structure. We started with a basic model with a fixed intercept, we then built models adding random intercept and slope, and finally an interaction term. Allowing intercepts to vary (i.e., random intercepts model) means that the outcome variable is at different levels prior to treatment (i.e., the symptom level varies across participants). Similarly, allowing slopes to vary means that the change trajectory may vary across participants. Finally, to investigate whether treatment condition moderated the effect, an interaction term of time (fixed effect; treatment period) and treatment condition (fixed effect; IUT or MCT) was added to the model with random intercept and slope. Each model’s fit to observed data was evaluated with the likelihood ratio test. Models with significantly better fit than previous models were retained. Standardized effect sizes for between-group effects at mid-treatment, post-treatment, and follow-up were calculated as Cohen’s Treatment response was assessed with the reliable change index [To assess recovery rates, two procedures were employed that combine statistically reliable change with clinically meaningful change. First, an RCI of 7 points and a cut-off of 53 points on the PSWQ as used in the study by van der Heiden and colleagues [