Measures

Gaming disorder (GD, last 6 months) was measured using the Game Addiction Scale (GAS) (7), which was translated into German and French for this study. The scale consists of seven Likert-type items with five response options ranging from 0 (never) to 4 (very often), and participants who responded to at least three items with a score of at least 2 (sometimes) were defined as presenting GD, as suggested by Lemmens and Valkenburg (7). Additionally, a continuous score as the sum of the seven items was used (ranging from 0 to 28). The wording of the GAS changed slightly between wave 1 and wave 3. In wave 1, the wording included, in addition to gaming, time spent on the internet (e.g., “Have you felt upset when you were unable to play or to spend time on the internet?”; italic part was added and differed from the original wording of the GAS). This was done, because at the time when the questionnaire for wave 1 was developed, it was thought that lots of games involve internet activities, and that GD may be impossible without spending time in the internet (online games). After the DSM-5 (13), released in 2013, included internet GD as a condition for further study, it became evident that gaming should subsequently be measured distinctly and not be mixed with time spent on the internet, and the original Game Addiction Scale (without adding reference to the internet in the wording of the questions) was therefore used in wave 3. To account for the differences in wording of the GAS in wave 1 and wave 3, to improve comparability across waves, and to reduce false positives, the GD scores of participants who did not play games at least weekly (and therefore may have a GAS score due to non-gaming related Internet use) were set to 0 in both waves. Cronbach's Alpha for the GAS scale was 0.895 in wave 1 and 0.868 in wave 3.

Adult attention deficit hyperactivity disorder (ADHD, last 12 months) was measured using the six-item screener version of the Adult ADHD Self-Report Scale (ASRS-v1.1) (34) developed by the World Health Organization (WHO) and based on the DSM-IV diagnostic criteria (35). Four items assessed the ADHD inattention subscale and two items assessed its hyperactivity subscale (see Table 2). Response options were on a five-point Likert-type scale ranging from 0 (never) to 4 (very often). For building a binary measure of ADHD, items were dichotomised—at least 2 (sometimes) for the first three items and at least 3 (often) for the last three items—and ADHD was defined as the presence of at least 4 symptoms as suggested by the authors of the scale (34). For analysis involving the continuous ADHD subscales of inattention and hyperactivity, the mean of the Likert-scale items (with values ranging from 0 to 4) was calculated. Cronbach's Alpha for the ADHD scale was 0.798 in wave 1 and 0.778 in wave 3.

Alcohol use disorder (AUD, last 12 months) was measured using 12 items for the 11 DSM-5 criteria (13, 36, 37) for AUD in a yes/no format. The DSM-5 moderate (4+) cut-off was used to define AUD. Cronbach's Alpha for the AUD scale was 0.729 in wave 1 and 0.696 in wave 3.

Cannabis use disorder (last 12 months) was measured using the revised version of the Cannabis Use Disorder Identification Test [CUDIT-R; (38), based on (39)]. The test consists of 8 five-point Likert-type items ranging from 0 (never) to 4 (daily or almost daily), a measure of the frequency of cannabis use ranging from 1 (monthly or less often) to 4 (four or more times per week), and one item with two response options, 0 (smoking cannabis for fun) or 4 (smoking cannabis out of habit). A cut-off of 8 out of 40 possible points was used to define cannabis use disorder. Cronbach's Alpha for the cannabis use disorder scale was 0.894 in wave 1 and 0.906 in wave 3.

Tobacco use disorder (last 12 months) was assessed using six items from the Fagerström Test for Nicotine Dependence (FTND (40). A cut-off of 3 out of 10 possible points was used to define tobacco use disorder. Cronbach's Alpha for the tobacco use disorder scale was 0.719 in wave 1 and 0.702 in wave 3.

Symptoms of major depression in the last 2 weeks were measured using the WHO's Major Depressive Inventory (41), consisting of 12 six-point Likert-type statements measuring 10 criteria and ranging from 0 (never) to 5 (always); two criteria were assessed using two statements each, with only the highest value of the two statements being used for the sum score. The sum of the criteria scores, ranging from 0 to 50, was used in this analysis. Cronbach's Alpha for the major depression scale was 0.889 in wave 1 and 0.888 in wave 3.

Mental health was assessed using the Medical Outcomes Study 12-Item Short Form Survey Instrument, v2 (SF-12) (42). The mental component summaries were linearly transformed into norm-based scores (mean = 50; SD = 10). Cronbach's Alpha for the SF-12 mental health scale was 0.772 in wave 1 and 0.790 in wave 3.

Life satisfaction was measured using the Satisfaction with Life Scale (43), consisting of five items with seven response options ranging from 1 (strongly disagree) to 7 (strongly agree). The sum of the items (ranging from 5 to 35) was calculated for the analysis. Cronbach's Alpha for the life satisfaction scale was 0.772 in wave 3. Life satisfaction was not measured in wave 1.

Poor performance at work/school was measured in wave 1 and wave 3 using a single question asking participants whether they had performed poorly at school or work, or got behind with work, in the last 12 months. Response options were from never to 10 or more times. This question was adapted from the ESPAD survey (44).

For all the scales used, missing values on single items were replaced by the scale mean. If more than 20% of the scale's items were missing, the scale was considered to be missing.