2.1 Measures
Well-being: Well-being was measured using the World Health Organisation-Five Well-Being Index (WHO-5; Allgaier et al., 2012; Brähler et al., 2007), containing five questions with total scores ranging from 0 (no well-being) to 100 (maximal well-being).
Depression: Depression symptoms were assessed using the Patient Health Questionnaire-9 (PHQ-9; Kroenke et al., 2001). The nine items ask about the last two weeks and yield a total score ranging from 0 to 27. The cut-off for clinically relevant symptoms is ≥11 for adolescents (Richardson et al., 2010). Item 9 of the PHQ-9 was used as an indicator of suicidal ideation (Rossom et al., 2017).
Suicidal Ideation: Item 9 of the PHQ-9 asks: “Over the last two weeks, how often have you been bothered by thoughts that you would be better off dead or of hurting yourself in some way?”. Response to this question was coded in a binary way to detect any recent suicidal ideas within the last two weeks (presence of suicidal thoughts = response to item 9 ranged from 1 to 3; absence of suicidal thoughts = response to item 9 was 0).
Anxiety : The Generalised Anxiety Disorder (GAD-7) scale was used to measure anxiety (Löwe et al., 2008). The seven items have a maximum score of 21 and the cut-off for clinically relevant anxiety symptoms is ≥11 in adolescents (Mossman et al., 2017).
Sleep: Insomnia and sleep quality were assessed with the Insomnia Severity Index (ISI; Gerber et al., 2016). The seven-item scale has a maximum score of 28 and a cut-off of ≥15 for moderate insomnia (Morin et al., 2011).
Gender was coded as girl, boy or non-binary. Each gender group was analysed separately because a) girls are over-represented in the sample, thus biasing the results of the full sample, and b) it is known that there are gender differences in mental health scores (Gilsbach et al., 2021).
Migration background: To assess the migration status, students were asked whether they and/or both parents were born abroad.