Results for the survey are based on telephone interviews conducted under the direction of Princeton Survey Research Associates among a nationwide sample of 1,513 adults, 18 years of age or older, during the period February 14-19, 2001. For results based on the total sample, one can say with 95% confidence that the error attributable to sampling and other random effects is plus or minus 3 percentage points. For results based on either Form 1 (N=728) or Form 2 (N=785), the sampling error is plus or minus 4.5 percentage points.
In addition to sampling error, one should bear in mind that question wording and practical difficulties in conducting surveys can introduce error or bias into the findings of opinion polls.
Survey Methodology in Detail
The sample for this survey was designed to generalize to the U.S. adult population in telephone households, and to allow separate analyses of responses by African-Americans and younger respondents. To achieve these objectives in a cost effective manner, the design uses standard list-assisted random digit dialing (RDD) methodology, but telephone numbers are drawn disproportionately from telephone exchanges with higher than average density of African-American households. Weighting adjustments are made in the analysis to ensure the overall representativeness of the sample.
Using RDD methods, every active block of telephone numbers (area code + exchange + two-digit block number) that contains one or more residential directory listings is proportionally likely to be selected; after selection two more digits are added randomly to complete the number. This method guarantees coverage of every assigned phone number regardless of whether that number is directory listed, purposely unlisted, or too new to be listed. After selection, the numbers are compared against business directories and matching numbers are purged.
The sample was 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. The use of replicates also insures that the regional distribution of numbers called is appropriate. Again, this works to increase the representativeness of the sample.
At least 10 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 is at home.” This systematic respondent selection technique has been shown empirically to produce samples that closely mirror the population in terms of age and gender.
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 2000). 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.
The weights are derived using an iterative technique that simultaneously balances the distributions of all weighting parameters.