Esther Mertens

| 49 Effectiveness of a Psychophysical Intervention conditions) using an online random number generator. Schools were stratified by school size (small to moderate sized schools with < 100 students in the 7th Grade, large schools with > 100 students in the 7th Grade) to enhance an equal distribution of students over the conditions. One school, allocated to the Control condition, dropped out after randomization and before the start of data collection due to a change in school management. This school was replaced by another school (see Figure 1 for the flow chart). Students completed digital questionnaires before the intervention started, at baseline (T1; October/November 2017), after completing the first year of R&W lessons (T2; March/April 2018), before the start of the R&W lessons in the second year of R&W (T3; October 2018), and after the intervention, at post measurement (T4; January 2019). These questionnaires were conducted by trained research assistants. Students gave active informed consent for completing the questionnaires. Parents gave passive informed consent for the participation of their child and active informed consent for their own participation. This trial was approved by the Ethical Committee of the Faculty of Social and Behavioral Sciences of Utrecht University (FETC17-05) and registered in the Dutch Trial Register, number NL6371 (NTR6554; see for protocol Mertens, Deković, Van Londen, & Reitz, 2018). Participants The sample consisted at baseline of 1299 7 th Grade students. In the Netherlands, 7 th Grade corresponds generally with ages 12 to 13 years. In our sample, the students had an average age of 12.38 years ( SD = .62). Of the students, 661 (54%) were boys and 815 (69%) had a Western background (see Table 1 for the demographics per condition). Table 1 Descriptives of Students’ Demographics at Baseline per Condition Light Standard Plus Control Differences at T1 F/χ 2 p η 2 partial /φ N 373 303 249 374 Age, M (SD) 12.33 (.57) 12.38 (.66) 12.34 (.60) 12.47 (.64) 3.89 .009 .009 Boys, n (%) 170 (48%) 161 (56%) 131 (55%) 199 (57%) 7.38 .061 .077 Western background, n (%) 291 (82%) 115 (43%) 211 (91%) 198 (59%) 182.01 <.001 .392 There were no differences between the conditions regarding sex distribution, but there were small differences in students’ age and ethnic background (see Table 1). Students in the Control condition were slightly older than students in the Light condition. Regarding ethnic background, the Control and Standard conditions had roughly an equal distribution of students with aWestern and non-Western background, 3