During the survey periods, we recruited a total of 10,075 students (3248 in 2008; 3386 in 2012; 3441 in 2016) enrolled in grades 10 through 12 (aged 15–18 years) in the study schools (29 schools in 2008 and 30 schools in 2012 and 2016). Among them, 8955 students participated in the surveys (513 declined to participate and 607 were absent from school). Of these, we excluded 1053 students with missing variables of interest. Finally, complete data on all variables of interest were available for 7902 students, which was 78.4% of the original sample (2528 in 2008; 2651 in 2012; 2723 in 2016), and these data were used for analysis.

The study was conducted in accordance with the Declaration of Helsinki, and the study protocols were approved by the Institutional Review Board of the University of the Ryukyus (No. 21, 139, 343).

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