This was a random telephone survey conducted by interviewers under close supervision. To minimize sampling bias, telephone numbers were first drawn randomly from the residential telephone directories as "seed numbers", from which another set of numbers was generated by computer, in order to capture the unlisted numbers. Duplicated numbers were then filtered, and the remaining numbers were mixed in random order to produce the final telephone sample.
The target population of this survey was Hong Kong citizens aged 15-24 above who spoke Cantonese. When telephone contact was successfully established with a target household, one eligible person was selected. If more than one subject had been available, selection was made using the "next birthday rule" which selected the person who had his/her birthday next. If the selected respondent were aged 15-17, parental consent would be sought at the beginning of questionnaire after the introduction.