PARTICIPANTS: The investigation was carried out from March to June 2000. The method of cohort stratified random sampling was adopted and the questionnaire was administered to adolescent students in Chongqing City and Lanzhou City.

• 对象:本调查于2000-03/06进行,采用整群分层随机抽样的方法,选取重庆市和甘肃省兰州市的青少年学生进行集体施测。