2 43 The effectiveness of Take it Personal! on substance use the original design (Schijven et al., 2015), we did not use life-time use of cannabis and illicit drugs (i.e. assessed with the item: ‘Have you ever used weed/illicit drugs (1) ‘yes’ (2) ‘no’) as primary outcomes. At baseline, 85% of our participants showed life-time use of cannabis and 58% showed life-time use of illicit drugs; examining change on these measures would not be useful. 2.4.3 Substance use severity To assess the severity of substance use, the Alcohol Use Disorders Identification Test (AUDIT; Babor et al., 2001) and the Drug Use Disorders Identification Test (DUDIT; Berman et al., 2003), as incorporated in the SumID-Q, were used. Each scale consisted of 10 items that could be rated on a five-point Likert scale, with answer categories ranging from (1) ‘never’ to (5) ‘almost every day’. The AUDIT and DUDIT items relate to frequency and quantity of use, dependency and problems related to use. An example is: ‘How often could you not stop drinking/drug use?’. The AUDIT and DUDIT have been shown to be applicable in people with a mild intellectual disability (van Duijvenbode et al., 2016). In the current sample, both AUDIT and DUDIT showed good internal consistency, with Cronbach’s α = 0.75 for the AUDIT and α = 0.81 for the DUDIT. 2.4.4 Binge drinking The frequency of binge drinking was assessed with one item from the Alcohol Use Disorders Identification Test (Babor et al., 2001) of the SumID-Q (VanDerNagel et al., 2011). Adolescents replied to the question: ‘How often do you drink more than six glasses on one occasion?’; the answer categories ranged from (1) ‘never’ to (5) ‘almost every day’. 2.5 Statistical analyses Sample size calculation was based on a previous personality-targeted intervention study with a medium effect size (Conrod et al., 2006). Power-analysis based on an average effect size of F = 0.25 (Conrod et al., 2006), a two-sided test at alpha = 0.05, a statistical power (1-β) of 0.80 and 10% loss-to follow-up after randomization. Based on these assumptions, a sample size of 140 adolescents was required (Schijven et al., 2015). In the Results section we elaborate on power and effect size calculations. Descriptive analyses were performed to examine baseline distributions of age, gender, total IQ and outcome measures among adolescents in the intervention and control groups. Because Take it Personal! was personalized and aimed to teach adolescents skills to reduce the most relevant substance(s), effectiveness was assessed for each adolescent’s most frequently or severely
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