Telephone interviews were conducted by interviewers under close supervision. All data were collected by our interviewers using a Computer Assisted Telephone Interview (CATI) system invented by the research team 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.
Telephone numbers are randomly generated using known prefixes assigned to telecommunication services providers under the Numbering Plan provided by the Office of the Communications Authority (OFCA). Invalid numbers are then eliminated according to computer and manual dialing records to produce the final sample.
The target population of this survey was Cantonese speaking working population in Hong Kong of age 25-50 who are MPF members. When telephone contact was successfully established with a target household, one eligible subject was interviewed. If more than one eligible subject had been available, selection was made using the "next birthday rule" which selected the person who had his/her birthday next. The survey was conducted during the period of November 17 to December 3, 2014. A total of 813 eligible subject were successfully interviewed. The effective response rate of this survey was 83.5%, and the standard sampling error for percentages based on this sample was less than 1.8 percentage points. In other words, the sampling error for all percentages using the total sample was less than plus/minus 3.5 percentage points at 95% confidence level.