Research Design返回


Research design


The target population of this survey was Cantonese-speaking population of Hong Kong aged 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 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. Please refer to Appendix 1 for the demographic profile of the respondents.


Telephone interviews were carried out from 1830 to 2230 hours between 28 February and 11 March 2003. 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 7,002 telephone numbers sampled for the survey, 2,160 were confirmed ineligible, among them 395 were fax or data lines, 1,157 were invalid telephone numbers, 61 were call-forwarding numbers, while another 284 were non-residential numbers. Besides, 118 of them was invalidated due to special technological circumstances, while 145 cases were voided because no eligible respondents were available at the numbers provided.


Meanwhile, a total of 2,563 telephone numbers were invalidated since the research team could not confirm their eligibility. Among them 75 were busy lines, 1,388 were no-answer calls, 20 cases were diverted to answering devices while 37 were blocked. In addition, 245 cases were treated as ineligible because of language problems, 444 interviews were terminated before the screening question, while 354 cases were voided for other problems.


Of the remaining 2,279 eligible cases, 1,254 failed to complete the interview. Among them 17 rejected the interview immediately after their eligibility was confirmed, 1,083 were unfinished cases with appointment dates beyond the end of fieldwork period. Besides, 90 cases were incomplete due to unexpected termination of interviews, 64 were classified as miscellaneous due to other non-contact problems, and the remaining 1,025 were successful cases (Table 1).

Table 1 Breakdown of contact information of the survey
   Frequency  Percentage
 Telephone numbers' ineligibility confirmed  2160   30.9% 
 Fax/ data line  395  5.6%
 Invalid number  1157  16.5%
 Call-forwarding/ mobile/ pager number  61  0.9%
 Non-residential number  284  4.1%
 Special technological circumstances  118  1.7%
 No eligible respondents  145  2.1%
 Telephone numbers' or respondents' eligibility not confirmed  2563   36.6% 
 Line busy  75  1.1%
 No answer  1388  19.8%
 Answering device  20  0.3%
 Call-blocking  37  0.5%
 Language problem  245  3.5%
 Interview terminated before the screening question  444  6.3%
 Others  354  5.1%
 Telephone numbers' eligibility confirmed, but failed to complete the interview  1254   17.9% 
 Household-level refusal  7  0.1%
 Known respondent refusal  10  0.1%
 Known respondent never available  0  0.0%
 Appointment date beyond the end of the fieldwork period  1083  15.5%
 Partial interview  90  1.3%
 Miscellaneous  64  0.9%
 Successful cases  1025   14.6% 
 Total  7002   100.0% 

To sum up, a total of 1,025 Hong Kong residents aged 18 or above were successfully interviewed in the survey. The effective response rate was 65.0% as shown in the following calculation. The standard error due to sampling was no more than 1.6 percentage points.

Table 2 Calculation of effective response rate

   Effective response rate
= [Successful cases / (Successful cases + Refusal cases +Incomplete cases)] 100%
= [1,025 / (1,025 + 17+ 534)] 100%
= 65.0%

The data collected have been adjusted according to the age and gender distributions of the Hong Kong population as reported in the 2001 Population Census. 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 universities 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.