This was a random telephone survey conducted by telephone 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 and random checking, 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.
The target population of this survey was the local working population of age 18 to 65 with a monthly personal income no less than $8,000. When telephone contact was successfully established with a target household, one target respondent 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 from all those present.
The fieldwork was conducted during the period of 14-19 January 2009. A total of 503 qualified respondents were successfully interviewed. As shown from the calculation in Appendix I, the overall effective response rate of this survey was 75.4% (Table 1), and the standard sampling error for percentages based on this sample was less than 2.2 percentage points. In other words, the sampling error for all percentages was less than plus/minus 4.5 percentage points at 95% confidence level. For detailed breakdowns of the contact information, please refer to Table 2 in Appendix I.