2.3. Sample Size
A sample size calculation was conducted using G*Power 3. To detect a significant interaction with at least a medium effect size, a minimal sample size of 17 participants was determined for each group. Because of an estimated 10% dropout and some attrition at measurements, the sample size was set at 20 participants in each group (Cohen f = 1.0) using general linear modeling with one within-subjects factor (two-time points) and one between-subjects factor (two interventions), and assuming a correlation of r = 0.5 between the repeated measures, an α-level of 0.05, and a power. Due to the COVID-19-related measures, it was necessary to end the planned data collection prematurely. Of 17 recruited participants, 4 were unable to complete the 3MDR intervention due to the COVID-19 measures. Two participants who completed the 3MDR therapy opted out of the study before completing follow-up data collection. The remaining participants (n = 11) completed 3MDR and subsequent follow-up measures up to 6 months post-intervention. Data from this sample was completed. Despite not reaching our initial sample size, the effect sizes were large enough to allow our analyses to detect statistically significant effects (see Results).