Data were derived from the 2006 Korean Youth Risk Behavior Web-based Survey (KYRBWS) conducted by the Korean Centers for Disease Control and Prevention (KCDC) to monitor the prevalence of high-risk behaviors. This study applied a stratified three-stage clustering design to produce a nationally representative sample of public and private middle and high school students in grades seven through twelve in all regions of Korea. The first stage involved 234 cities and districts with similar characteristics in regard to the degree of geographic accessibility; numbers of schools and residents, and employment rate in general and in the farming, mining, and service industries in particular. In the second stage, 400 middle schools and 400 high schools were selected from within the 234 cities and districts. The third stage of sampling consisted of randomly selecting one class from each grade within each chosen school. All students in selected classes were eligible to participate. The survey proceeded as follows: during July, school teachers were trained to assist students with participating in the survey; during August, the KCDC informed each school about the schedule for
