The surveys are based on telephone interviews conducted under the direction of Princeton Survey Research Associates among nationwide samples. The samples for these surveys are random digit samples of telephone numbers selected from telephone exchanges in the continental United States. The random digit aspect of the sample is used to avoid “listing” bias and provides representation of both listed and unlisted numbers (including not-yet-listed). The design of the sample ensures this representation by random generation of the last two digits of telephone numbers selected on the basis of their area code, telephone exchange, and bank number.
The telephone exchanges were selected with probabilities proportional to their size. The first eight digits of the sampled telephone numbers (area code, telephone exchange, bank number) were selected to be proportionally stratified by county and by telephone exchange within county. That is, the number of telephone numbers randomly sampled from within a given county is proportional to that county’s share of telephone numbers in the U.S. Only working banks of telephone numbers are selected. For the Standard sample, a working bank was defined as 100 contiguous telephone numbers containing three or more residential listings. For the Rigorous sample, a working bank was defined as 100 contiguous telephone numbers containing one or more residential listings.
The samples were released for interviewing in replicates. Using replicates to control the release of sample to the field ensures that the complete call procedures are followed for the entire sample.
There were several differences in the way the Standard and the Rigorous surveys were administered:
The Standard survey was conducted June 18-22, 1997. At least five attempts were made to complete an interview at every sampled telephone number. The calls were staggered over times of day and days of the week to maximize the chances of making a contact with a potential respondent. All interview breakoffs and refusals were re-contacted at least once in order to attempt to convert them to completed interviews. In each contacted household, interviewers asked to speak with the “youngest male 18 or older who is at home”. If there is no eligible man at home, interviewers asked to speak with “the oldest woman 18 or older who lives in the household”. This systematic respondent selection technique has been shown empirically to produce samples that closely mirror the population in terms of age and gender. Quotas of approximately 50 percent male respondents and 50 percent female respondents were imposed for the sample.
The Rigorous survey was conducted June 18 through August 12, 1997. Households in the Rigorous sample with listed telephone numbers — for whom a mailing address could be obtained — were sent an advance letter asking for their participation in the survey. A $2 bill was enclosed with this letter as an additional incentive. There was no limit on the number of attempts to complete an interview at every sampled telephone number — numbers were called throughout the survey period until an interview was completed. The calls were staggered over times of day and days of the week to maximize the chances of making a contact with a potential respondent. A random selection procedure was used to select the respondent to be interviewed in each household. In addition, all interview breakoffs and refusals were contacted up to two additional times in order to attempt to convert them to completed interviews. For households with a known mailing address, respondents who refused to be interviewed after two calls were sent a conversion letter by priority mail before they were called a third time.
Non-response in telephone interview surveys produces some known biases in survey-derived estimates because participation tends to vary for different subgroups of the population, and these subgroups are likely to vary also on questions of substantive interest. In order to compensate for these known biases, the sample data are weighted in analysis. The demographic weighting parameters are derived from a special analysis of the most recently available Census Bureau’s Current Population Survey (March 1996). This analysis produced population parameters for the demographic characteristics of households with adults 18 or older, which are then compared with the sample characteristics to construct sample weights. The analysis only included households in the continental United States that contain a telephone. In addition to the demographic weighting parameters, the weighting for the Rigorous sample is also adjusted for the number of adults and number of telephone lines in each household. The weights are derived using an iterative technique that simultaneously balances the distributions of all weighting parameters.
The following table presents the full disposition of sampled telephone numbers for the Standard and Rigorous surveys: