Research DesignBack


 
 

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.

 
 

Telephone interviews were carried out from 1830 to 2230 hours between 4 and 9 January 2002. 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, amongst the 7,159 telephone numbers sampled for the survey, 911 were found to be invalid lines, 1,702 were non-contacts, including busy lines and no answer, another 319 were business lines, 512 were fax lines or with answering machines, while 124 were classified as miscellaneous due to other non-contact problems. Of the 3,591 valid contacts, the numbers of refusals (without knowing whether target respondents were qualified or not), disqualified and unfinished cases with appointment dates beyond the end of fieldwork period were 741, 262 and 384 respectively. Besides, 67 cases were incomplete due to unexpected termination of interviews, 631 have language and other problems, and the remaining 1,506 were successful cases.

 
 
Table 1. Breakdown of contact information of the survey
   Frequency  Percentage
 Telephone numbers attempted  7,159   
 Invalid numbers  911  
 Busy / no answer  1,702  
 Business lines  319  
 Fax / answering machine  512  
 Miscellaneous  124  
     
 Valid contacts  3,591   100.0% 
 Refusal  741  20.6%
 Failed to qualify  262  7.3%
 Appointment  384  10.7%
 Incomplete  67  1.9%
 Other problems  631  17.6%
 Successful  1,506  41.9%
 
 

To sum up, a total of 1,506 Hong Kong residents aged 18 or above were successfully interviewed in the survey. The effective response rate was 65.1% as shown in the following calculation. The standard sampling error for percentages based on this sample was less than 1.3 percentage points.

 
 
Table 2 Calculation of effective response rate

   Effective response rate
= [Successful cases / (Successful cases + Refusal cases +Incomplete cases)] 100%
= [1,506 / (1,506 + 741+ 67)] 100%
= 65.1%
 
 

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 were based on the weighted data.