64 Chapter 3 3.2 Intervention effect on behavioral problems Table 3 presents the effects of Take it Personal! on anxious, withdrawn, rulebreaking, and aggressive problems. Adolescents and young adults who followed Take it Personal! showed less rule-breaking problems at follow-up compared to the adolescents and young adults in the control condition (B = -1.15, SE B = 0.55, p = 0.041). This finding reflects a small effect size with Cohen’s d = 0.21. There were no significant effects of Take it Personal! on anxious, withdrawn and aggressive problems. 3.3 Moderating effects of behavioral problems on substance use Table 4 shows that anxious, withdrawn, rule-breaking, and aggressive problems did not moderate the program’s effect on substance use frequency, as there were no significant interactions between condition and the different emotional and behavioral problem domain scores at baseline. 3.4 Power Participant recruitment in this complex target group proved difficult, resulting in a sample size (N = 66) that was smaller than intended. A-priori power analysis based on a medium effect size for main target substance use frequency, initially suggested that a sample size of 140 adolescents and young adults was necessary for an 80% chance of finding an effect on substance use frequency at α = 0.05 (Schijven et al., 2015). Although the effectiveness on substance use frequency had a medium effect size (Schijven et al., 2020a), the effectiveness on rule-breaking had a small effect size. Post-hoc power analysis estimates that with the current sample size we only had an 51% chance of finding this effect for rule-breaking problems.
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