Maartje Boer

INTRODUCTION 15 1 SMU problems, and wellbeing remain, which we discuss below and aim to answer in this dissertation. How Should We Measure Problematic SMU? Although research on problematic SMU has grown in the past decade, validation research on instruments measuring problematic SMU lags behind. Several instruments have been developed, yet most of them have not been subjected to validation (Andreassen, 2015). Currently, only one instrument has been validated in nationally representative adolescent samples (Andreassen et al., 2016; Bányai et al., 2017; Lin et al., 2017). This instrument measures problematic SMU based on six criteria of addiction mentioned above, including preoccupation (i.e., salience), escape (i.e., mood modification), tolerance, withdrawal, relapse (i.e., persistence), and conflict (Andreassen et al., 2016; Griffiths, 2005). However, extending the measurement with additional items that measure negative consequences due to SMU possibly advances the measurement of problematic SMU, as problematic SMU refers to addiction-like SMU, and one of the core aspects of behavioral addiction is that the behavior in question impairs daily life (Kardefelt-Winther et al., 2017; Van Rooij et al., 2018). To facilitate future research on problematic SMU, it is essential that it is measured with a scale that encompasses all theoretical criteria of problematic SMU and furthermore, that this scale is found to be reliable and valid within a nationally representative sample. Future research on problematic SMU is considered important, given the growing evidence that problematic SMU is negatively associated with adolescent wellbeing (Marino et al., 2018b). Furthermore, a scale with good psychometric properties may provide health professionals with a psychometrically sound instrument to screen adolescents for problematic SMU. In addition, there is no scale that measures problematic SMU for which the reliability and validity has been assessed and compared across different countries. Furthermore, it has not been investigated whether adolescents fromdifferent countries interpret thequestions fromaproblematicSMU-scale in the same way, and thus whether the scale measures the same underlying construct cross-nationally. A valid and reliable instrument with equivalent interpretations across various national settings provides researchers worldwide with an instrument to accurately measure problematic SMU