Maartje Boer

VALIDATION OF THE SMD-SCALE 33 2 conduct problems, hyperactivity, and peer problems (Goodman et al., 1998) . Each subscale consists of five items, for example “I worry a lot” (emotional problems), “I am often accused of lying and cheating” (conduct problems), “I am easily distracted, I find it difficult to concentrate” (hyperactivity), and “Other children or young people pick on me or bully me” (peer problems). Answer categories were (0) not true , (1) somewhat true, and (2) certainly true. Given the ordinal nature of the items, internal consistency of the test scores of each adapted subscale was calculated using the ordinal alpha based on the polychoric correlationmatrix (Gadermann et al., 2012). Ordinal alpha was 0.81 for emotional problems, 0.67 for conduct problems, 0.76 for hyperactivity, and 0.64 for peer problems. Our aimwas to study the associations between problematic SMU and problematic levels of mental health problems. Therefore, subscale sum-scores were dichotomized in line with recommendations fromGoodman and colleagues (1998): Subscale sum-scores higher than the 80 th centile were coded as (1) borderline or abnormal , whereas subscale sum-scores lower than the 80 th centile were coded as (0) normal . School Problems Adolescents were asked how they feel about school at present, with response ranging from (1) I like it a lot to (4) I don’t like it at all (Inchley et al., 2016) . In order to study associations with particularly school dissatisfaction, the variable was recoded into a dichotomous variable school dissatisfaction, with categories (1) I don’t like it very much/I don’t like it at all and (0) I like it a lot / I like it a bit . Adolescents were also asked whether they feel pressured by the schoolwork they have to do, with responses ranging from (1) not at all to (4) a lot (Inchley et al., 2016) . To study the association between problematic SMU and schoolwork pressure, this variable was dichotomized into the variable perceived school pressure , with categories (1) some/a lot and (0) not at all/a little . Sleep Problems Adolescents were asked what time they usually go to bed and what time they usually wake up on schooldays. Answers on these questions were used to establish whether the reported average sleep duration met the age- specific recommendation for daily sleep duration according to the National