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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.
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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.
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Telephone interviews were conducted during the period of 9 to 12 December 2005. A total of 511 Cantonese-speaking Hong Kong citizens aged 18 or above were successfully interviewed. The overall response rate of this survey was 70.7% (Table 2), 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 4.4 percentage points at 95% confidence level.
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