This was a random telephone survey conducted by telephone 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 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 Cantonese-speaking population of Hong Kong aged 18 or above. When telephone contact was successfully established with a target household, one person aged 18 or above 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.
Telephone interviews were conducted during the period of 23 to 25 June 2003. A total of 1,032 Cantonese-speaking Hong Kong citizens aged 18 or above were successfully interviewed. As shown from the calculation below, the overall response rate of this survey was 65.2% (Table 1), and the standard sampling error for percentages based on this sample was less than 1.6 percentage points. In other words, the sampling error for all percentages was less than plus/minus 3.2 percentage points at 95% confidence level.
* Including "partial interview" and "interview terminated before the screening question"
As shown in Table 2, among the 5,833 telephone numbers sampled for the survey, 1,776 were confirmed to be ineligible, among them 328 were fax or data lines, 1,001 were invalid telephone numbers, 33 were call-forwarding numbers, while another 226 were non-residential numbers. Besides, 92 of them were invalidated due to special technological reasons, while 96 cases were voided because target respondents were unavailable at the numbers provided.
Meanwhile, a total of 1,731 telephone numbers were invalidated before the research team could confirm their eligibility. Among them 71 were busy lines and 952 were no-answer calls after making a maximum of 5 times' recalls. Twenty-six cases were diverted to answering devices while another 31 were blocked. Moreover, 121 cases were treated as unsuccessful because of language problems, while 435 interviews were terminated before the screening question. Ninety-five cases were voided for other problems.
On the other hand, 1,294 cases failed to complete the interview. Among them 8 were rejected at the household level, another 15 rejected the interview immediately after their eligibility was confirmed, 1,039 were unfinished cases with appointment dates beyond the end of fieldwork period. Besides, 94 cases were incomplete due to unexpected termination of interviews, 138 were classified as miscellaneous due to other non-contact problems, and the remaining 1,032 were successful cases (Table 2).