Maartje Boer

CHAPTER 3 62 involved sampling via randomly selected schools and classes (Inchley et al., 2018). Surveys were administered in classroom settings during school hours through digital (45%) or paper-and-pencil (55%) self-completion. Respondents were informed that participation was voluntary and anonymous. Active informed consent was obtained fromschools and participants. Depending on the country, passive or active informed consent was obtained from parents. Participating countries obtained ethical approval of the study procedures from their institutional ethics committee (Inchley et al., 2018). Measures Problematic SMU Problematic SMU was assessed with the 9-item SMD-scale (Van den Eijnden et al., 2016). The questions were introduced with ‘We are interested in your experiences with social media. The term social media refers to social network sites (e.g., Facebook, [add other local examples]) and instant messengers (e.g., [insert local examples], WhatsApp, Snapchat, Facebook messenger).’ Subsequently, respondents were asked ‘During the past year, have you…’, followed by, for example, ‘regularly found that you can’t think of anything else but the moment that you will be able to use social media again?’ (preoccupation), with answer options 1 yes and 0 no . All items can be found in the Appendix (Table A3.1). For the criterion validity analyses, the sum- score of the scale was dichotomized, whereby adolescents reporting six to nine present symptoms were defined as a problematic user (1 problematic user: 6-9 symptoms, 0 non-problematic user: 0-5 symptoms ) (Boer, Stevens, Finkenauer, Koning, et al., 2021; Boer, Van den Eijnden, et al., 2020). This definition is based on a latent class analysis on the nine items in a nationally representative sample of Dutch adolescents aged 12-16, which identified three subgroups of users, whereby adolescents in the subgroup with the highest levels of problematic SMU reported six or more symptoms (Boer, Stevens, Finkenauer, Koning, et al., 2021). Mental Wellbeing We assessed two indicators ofmental wellbeing. Life satisfaction wasmeasured using the Cantril ladder, where respondents rated their life on a scale, ranging