Methods
Sample and Setting
Participants were recruited from 36 middle schools and 23 high school among the 655 middle and high schools in Seoul , Korea , by school nurses who were informed about the study . A convenience sample of 19,924 student participated in the study. Exclusion of participants with incomplete data resulted in a final sample of 18,752 adolescents , consisting of 11,295 middle school student and 7,457 high school The study was approved by the human subject is protection committee of the Seoul Metropolitan office of Education and principals from each school. Data were collected using a self-report questionnaire during April to June 2006. All participants and their parents agreed to participate and were informed about opportunities to withdraw and that there would be no disadvantages from the withdrawal. Anonymity and confidentiality were guaranteed as usual.