Fifteen secondary schools were selected from the pool of the BOKSS school network and were successfully contacted by BOKSS. Consent to participate in this voluntary study was obtained from the school principals as well as parents of the students coming from the selected classes within each school.

In details, POP had randomly selected two classes for each of the fifteen schools by means of a computer program and all students of these two classes would be invited to take part in this survey. In case any of the first two selected classes was not available, a backup class (i.e. the third one) would be used according to the random order list generated by the computer.

After selecting the target classes, self-administered paper questionnaires were sent to all fifteen schools waiting for dispatch. Letters to parents were distributed to the students involved before the data collection started so as to inform them of the purpose and procedures of the survey. Parents who refused to allow their children to participate in the survey could return a reply slip to the school.

With their parental consent, students in the target classes were asked to fill in the survey questionnaires in class and return to the teacher in-charge on the spot. In other words, no questionnaires were allowed to take away from school.