Methods
Data source
The data used in this study were obtained from the BSS for HIV/AIDS of the BMA. The adolescents in grade 11 of high schools and the second year of vocational colleges were recruited in this survey every year from 2002 to 2006. These two groups were chosen because of known differences in their sexual activity. Vocational college students are more sexually active than the high school students . From 2002 to 2005 the data were collected in January using self-administered questionnaires. In 2006, the survey was conducted in June using a web-based question- naire