Maartje Boer

SMU AND MENTAL HEALTH 161 6 as in T1 (45.0% girls, 23.7% immigrant background, 62.5% pre-vocational, 26.2% intermediate, and 11.3% pre-university). In T3, two pre-vocational level schools dropped out due to practical circumstances, as well as several pre-vocational level classes from the other schools, which yielded a different sample composition compared to T1 and T2 (43.9% girls, 17.6% immigrant background, 33.8% pre-vocational, 44.6% intermediate, and 21.6% pre-university). Two weeks prior to the survey assessment, information letters were sent to parents to provide information about the survey and to allow parents to refuse participation of their child. One week prior to the survey assessment, students were informed about the subject and purposes of the study, that participation was voluntary and anonymous, and that they could resign participation at any moment. Students completed the online survey during school hours. Research-assistants monitored students’ survey completion and provided help where necessary. The study procedures adhered to the Declaration of Helsinki and were approved by the ethical board of the Faculty of Social Sciences at Utrecht University (FETC16-076 Eijnden). Measures SMU Problems SMU problems were measured using the 9-item Social Media Disorder-Scale, that assesses nine symptoms of addiction to social media (Van den Eijnden et al., 2016). Respondents were asked ‘During the past year, have you …’, followed by ‘regularly found that you can’t think of anything else but the moment that you will be able to use social media again’ (preoccupation), ‘regularly felt dissatisfied because you wanted to spend more time on social media’ (tolerance), ‘often felt bad when you could not use social media’ (withdrawal), ‘been unable to stop using social media, even though others told you that you really should’ (persistence), ‘regularly had no interest in hobbies or other activities because you would rather use social media’ (displacement), ‘regularly had arguments with others because of your social media use’ (problem), ‘often used social media secretly’ (deception), ‘often used social media so you didn't have to think about unpleasant things’ (escape), and ‘had serious conflict with your parent(s) and sibling(s) because of your social media use (conflict), with a dichotomous response scale ( yes or no ). The scale has