Maartje Boer

SUMMARY AND DISCUSSION 287 9 educational levels. Three subgroups of adolescents with different patterns of SMU problems were identified: A normative (i.e., reporting no symptoms or one symptom), at-risk (i.e., reporting two to five symptoms), and problematic (i.e., reporting six to nine symptoms) group. Higher scores on the SMD-scale were associated with a greater probability of reporting problems related to mental health, school, and sleep, which confirmed criterion validity. Together, these findings indicate that the SMD-scale is suitable for assessing problematic SMU among Dutch adolescents. There is no scale that measures problematic SMU for which its psychometric properties have been investigated and compared across various national settings. Building on the previous chapter, Chapter 3 validated the SMD-scale using large-scale cross-sectional nationally representative data from 222,532 adolescents from 44 countries within the European region and Canada. Results confirmed good structural validity of a unidimensional scale in all countries. Also, the internal consistency of the items was good in all countries, indicating that the scores on the scale are reliable. In addition, the factor structure of the scale was measurement invariant across countries, gender, socioeconomic status, and age, with the exception that age invariance was not established in one country. In almost all countries, problematic SMU was positively related to higher SMU intensity, indicated by the intensity of online communication, and negatively with mental health 6 , indicated by life satisfaction and psychosomatic complaints, suggesting appropriate criterion validity. Overall, the study demonstrated that the SMD-scale has good psychometric properties in many national contexts, facilitating future (cross- national) research on problematic SMU. Studies on the association between SMU behaviors and wellbeing typically rely on single-country data, leaving the question whether these associations are country-specific unanswered. Chapter 4 focused on the question whether intense SMU (i.e., engaging in online communication throughout the whole day) and problematic SMU and their associations with mental 7 , school, and social wellbeing indicators depended upon the country context using international cross-sectional data. Furthermore, it examined whether differences in intenseandproblematicSMUacross countries couldbe 6 In Chapter 3, we referred to mental wellbeing instead of mental health. 7 In Chapter 4, we referred to mental wellbeing instead of mental health.