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The target population of this survey was Cantonese-speaking population of Hong Kong of age 18 or above. To minimize sampling bias, the following sampling technique was adopted:
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.
When telephone contact was successfully established with a target household, one person of age 18 or above was selected. If more than one qualified 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. Please refer to Appendix 1 for the demographic profile of the respondents.
Telephone interviews were carried out between 25 May and 1 June 2007. Data were collected by interviewers using a Computer Assisted Telephone Interviews (CATI) system under close supervision.
As shown from the detailed breakdown of the contact information, among the 11,919 telephone numbers sampled for the survey, 4,869 were confirmed ineligible, among them 627 were fax or data lines, 3,394 were invalid telephone numbers, 55 were call-forwarding numbers, while another 617 were non-residential numbers. Besides, 95 of them were invalidated due to special technological circumstances, while 81 cases were voided because no eligible respondents were available at the numbers provided.
Meanwhile, a total of 3,189 telephone numbers were invalidated since the research team could not confirm their eligibility. Among them 151 were busy lines, 1,985 were no-answer calls, 46 cases were diverted to answering devices while 53 were blocked. In addition, 200 cases were treated as ineligible because of language problems, 723 interviews were terminated before the screening question, while 31 cases were voided for other problems.
Of the remaining 3,861 eligible cases, 2,651 failed to complete the interview. Among them 15 rejected the interview immediately after their eligibility was confirmed, 2,510 were unfinished cases with appointment dates beyond the end of fieldwork period. Besides, 38 cases were incomplete due to unexpected termination of interviews, 88 were classified as miscellaneous due to other non-contact problems, and the remaining 1,210 were successful cases (Table 1).
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| Table 1 - Breakdown of contact information of the survey
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| Telephone numbers' ineligibility confirmed |
4,869 |
40.9 |
| Fax/ data line |
627 |
5.3 |
| Invalid number |
3,394 |
28.5 |
| Call-forwarding/ mobile/ pager number |
55 |
0.5 |
| Non-residential number |
617 |
5.2 |
| Special technological circumstances |
95 |
0.8 |
| No eligible respondents |
81 |
0.7 |
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| Telephone numbers' or respondents' eligibility not confirmed |
3,189 |
26.8 |
| Line busy |
151 |
1.3 |
| No answer |
1,985 |
16.7 |
| Answering device |
46 |
0.4 |
| Call-blocking |
53 |
0.4 |
| Language problem |
200 |
1.7 |
| Interview terminated before the screening question |
723 |
6.1 |
| Others |
31 |
0.3 |
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| Telephone numbers' eligibility confirmed, but failed to complete the interview |
2,651 |
22.2 |
| Household-level refusal |
5 |
0.0 |
| Known respondent refusal |
10 |
0.1 |
| Appointment date beyond the end of the fieldwork period |
2,510 |
21.1 |
| Partial interview |
38 |
0.3 |
| Miscellaneous |
88 |
0.7 |
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| Successful cases |
1,210 |
10.2 |
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| Total |
11,919 |
100.0 |
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The data collected have been adjusted according to provisional figures obtained from the Census and Statistics Department regarding the gender-age distribution of the Hong Kong population at the end of 2006. All analyses in this report are based on the weighted data.
Statistical tests of "difference-of-proportions" and "difference-of-means" have been employed whenever applicable, in order to check for significant changes. Figures marked with double asterisks (**) indicated that the variation has been tested to be statistically significant at p=0.01 level, whereas those with single asterisk (*) denoted statistical significance at p=0.05 level.
The researcher is aware that the POP Team is part of the University of Hong Kong, which is one of the institutions rated by the respondents. As a precaution to eliminate any possible bias due to desirability effect, all respondents were explicitly told at the beginning of the interview that the POP Team was an independent research team, and the respondents should simply report honestly what they felt.
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