This was a random telephone survey conducted by real interviewers under close supervision. All data were collected by our interviewers using a Computer Assisted Telephone Interview (CATI) system which allowed real-time data capture and consolidation. To ensure data quality, on top of on-site supervision, voice recording, screen capturing and camera surveillance were used to monitor the interviewers' performance.
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 using the "plus/minus one/two" method, 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.
Target respondents of the study were Cantonese-speaking population of Hong Kong of age 15 or above. When telephone contact was successfully established with a target household, only one qualified person from the household was selected using the "next birthday rule". If the selected subject was aged below 18, the interviewer first introduced the survey to his/her parent or guardian and sought his/her consent before interviewing the subject.
To test the validity of the questionnaire and the time required to complete the interview, a pilot test was conducted between January 6 and 7, 2009, and a total of 20 local citizens of age 18 or above were interviewed. Both the length and some wordings of the questions were slightly fine-tuned according to the comments and results collected from this pilot study.
The official fieldwork was conducted during the period of January 14 to 22, 2009. A total of 1,531 qualified local citizens were successfully interviewed. The overall response rate was 64.3% (Table 2), and the sampling error for percentages was less than 1.3%. In other words, the sampling error for all percentages was less than plus/minus 2.6% at 95% confidence level.
To ensure representativeness of the findings, the raw data collected have been weighted according to provisional figures obtained from the Census and Statistics Department regarding the gender-age distribution of the Hong Kong population in mid-year 2008. All figures in this report are based on the weighted sample.