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.