208 Chapter 4 Internalising and Externalising Problems Children’s internalising and externalising problems were measured with the mother version of the Strengths and Difficulties Questionnaire (SDQ; Goodman, 1997; Stone et al., 2010). The SDQ includes 25 items divided in three subscales: (1) internalising problems: emotional symptoms and peer relationship problems; (2) externalising problems: conduct problems and hyperactivity/ inattention symptoms; (3) prosocial behaviour. We used this three subscales division, because it is more valid than the original five subscales division in a low-risk (i.e., without disorders) sample (Goodman et al., 2010). We only used the first two subscales in this study. All items were rated on a three-point scale: 0 = not true, 1 = somewhat true, 2 = certainly true. The two subscales showed good convergent and discriminant validity, and good internal reliability (Goodman et al., 2010). In our sample, Cronbach’s alphas were between 0.72–0.75 for internalising problems and between 0.75–0.79 for externalising problems across all time points. Two outcome variables were computed: internalising problems and externalising problems. Maternal Mental Health Problems: Depression, Anxiety and Stress Maternal mental health problems were assessed with the 21 items version of the Depression Anxiety Stress Scales (DASS-21; Antony et al., 1998). The 21 items, all covering negative feelings, were rated on a four-point scale (0 = not at all, 1 = sometimes, 2 = often, 3 = usually) and represented three subscales: depression, anxiety, and stress. The subscales show good concurrent validity and reliability (Antony et al., 1998). In our sample, Cronbach’s alphas were 0.89 for depression, 0.81 for anxiety and 0.87 for stress at pre-test. Three outcome predictor variables were computed: maternal depression, maternal anxiety and maternal stress. Data Analyses A t-test and a χ2-test were performed in IBM SPSS Statistics 23 to assess whether randomisation was successful for sex and age. To test non-inferiority, we used a two-sided confidence interval (CI) approach. The idea behind this approach is that if the upper bound of the CI for the difference in mean change in secondary outcomes is below the margin of non-inferiority, MindLight is non-inferior to CBT. Based on a previous indicated anxiety prevention trial (Van Starrenburg et al., 2017), the margin of non-inferiority was set at 0.38 SEQ points for social, emotional and academic self-efficacy, at 1.11 SDQ points for internalising problems, and at 0.90 SDQ points for externalising symptoms. These differences correspond to 0.5 SD of the change in emotional
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