Maartje Boer

CHAPTER 8 260 (i.e., not due to selective dropout), but to classes and schools dropping out. Also, we aimed to limit any potential bias by imputing missing data based on available data at all waves (Madley-Dowd et al., 2019). Conclusion Findings from this study showed that at the within-person level, on average, changes in adolescents’ intensity of engagement in SMU activities were not associated with changes in their wellbeing (i.e., life satisfaction). However, across adolescents, these within-person associations ranged fromnegative to positive, suggesting that SMUcanbebeneficial aswell asharmful towellbeing. At the between-person level, a higher intensity of some SMU activities was associated with lower wellbeing. However, these associations were small and disappeared when controlling for SMU problems. Thus, these negative associations were explained by SMU problems rather than by adolescents’ SMU intensity. The results imply that considering individual differences, distinguishing SMU intensity from SMU problems, and disentangling within- from between-person effects are crucial for understanding the association between adolescents’ SMU intensity and their wellbeing.