Maartje Boer

CROSS-NATIONAL VALIDATION OF THE SMD-SCALE 63 3 from 0 worst possible life to 10 best possible life (Cantril, 1965). This measure has shown good test-retest reliability and (cross-national) convergent validity with other mental wellbeingmeasures (Casas & Rees, 2015; Levin & Currie, 2014; Mazur et al., 2018). Psychosomatic complaints weremeasured using the 8-item HBSC Symptom Checklist (Haugland & Wold, 2001). Respondents were asked how often in the past six months they had experienced, for example, feeling low (psychological complaint), or headache (somatic complaint), with answer options ranging from 1 about every day to 5 rarely or never . A mean score was computed after scores were rescaled, such that high scores indicate high levels of psychosomatic complaints (range: 1-5). Validation studies on the 8-item measure have shown adequate test-retest reliability, good content validity, and high factor loadings ( > 0.50) across different national settings (Haugland & Wold, 2001; Ravens-Sieberer et al., 2008). Intensity of Online Communication A newly developed 4-item measure, adapted from the EU Kids Online Survey on the frequency of online communication with different contacts (Mascheroni & Ólafsson, 2014), was used. Respondents were asked how often they have online contact through social media with close friends, friends from a larger friend group, friends they met through internet, and other people (e.g., parents, siblings, classmates, teachers). Answer options ranged from 1 never/almost never to 5 almost all the time throughout the day, and a don’t know/doesn’t apply option. The intensity of online communication was defined by the maximum score of the four items. Hence, higher scores indicate higher intensity of online communication (range: 1-5). Demographic Characteristics Gender was assessed by asking respondents whether they are boy or girl (1 girl , 0 boy ). Age was computed based on the respondent’s month and year of birth and the date of the survey assessment. For themeasurement invariance analysis, respondents were assigned to three categories: 11- ( ≥ 10 and ≤ 12.5 ), 13- ( > 12.5 and ≤ 14.5 ), and 15-year-olds ( > 14.5 and ≤ 16.5 ). Socioeconomic status was measured with the 6-item Family Affluence Scale (FAS) (Currie et al., 2008), which assesses material assets in the household (e.g., number of cars).