Maartje Boer

CHAPTER 5 140 detection of moderate effects ( β = 0.3), power ranged between 0.94 and 1.00 for all estimates. For detection of small effects ( β = 0.2) power ranged between 0.68 and 0.94 for all estimates. We could not derive the minimum relevant effect size from the literature, because no longitudinal studies specifically focusing on social media behaviors and ADHD-symptoms exist. Cross-sectional studies examining the relation between (problematic) SMU and ADHD-symptoms using multivariate models showed small-to-medium effect sizes with β = 0.24 on average (Andreassen et al., 2016; Barry et al., 2017; Levine et al., 2007; Mérelle et al., 2017; Wu et al., 2013) . For this effect size, power ranged between 0.80 and 0.99 for all estimates in our model. Thus, the analyses showed that our sample size of n = 543 is able to detect effect sizes corresponding to previous cross-sectional studies. Results Preliminary Results Prior to the main analysis, preliminary analyses were conducted on the data in long format ( n = 1,629) to study the intra-class correlations (ICC’s) of our measures. ICC’s express the proportion of variance that is explained on the between-person level relative to the total variance, which provides insight into the stability of our measures over time. Table 5.3 shows that the majority of the measures in our study varied mainly between adolescents, especially for SMU problems (89.9%). This means that most of our measures were relatively stable over time. However, a substantial part of the variance of our measures was related to changes within adolescents over time (10.1% to 28.5%). We also studied how our measures developed over time and whether the measures were related with demographic characteristics by means of multilevel multiple regression ( n = 1,629). Table 5.3 shows that on average, SMU intensity, SMU problems, and attention deficits increased in the second and the third wave relative to the first wave. Impulsivity only increased in the third wave relative to the first wave. Hyperactivity did not increase over time, although the ICC indicated that hyperactivity varied across waves Girls reported