Results for this survey are based on telephone interviews conducted under the direction of Princeton Survey Research Associates International among a nationwide sample of 1,708 adults, 18 years of age or older, from January 10-15, 2007. For results based on the total sample, one can say with 95% confidence that the error attributable to sampling is plus or minus 3 percentage points. For results based on Form 1 (N=863) and Form 2 (N=845) the sampling error is plus or minus 4 percentage points. For results based on January 11-15, 2007 (N=1,384), the sampling error is plus or minus 3.5 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.
This survey includes an oversample of 395 respondents in congressional districts that switched party control in the 2006 election. Flipped districts were identified using the 2006 House Election Results from the Cook Political Report, published December 13, 2006. In all, there were 30 congressional districts that switched hands: Arizona-05, Arizona-08, California-11, Colorado-07, Connecticut-02, Connecticut-05, Florida-16, Florida-22, Indiana-02, Indiana-08, Indiana-09, Iowa-01, Iowa-02, Kansas-02, Kentucky-03, Minnesota-01, New Hampshire-01, New Hampshire-02, New York-19, New York-20, New York-24, North Carolina-11, Ohio-18, Pennsylvania-04, Pennsylvania-07, Pennsylvania-08, Pennsylvania-10, Texas-22, Texas-23 (Runoff election was held on Dec. 12, 2006), Wisconsin-08. The oversample has a sampling error of plus or minus 6 percentage points.
Survey Methodology in Detail
The sample for this survey is a random digit sample 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. A working bank is defined as 100 contiguous telephone numbers containing one or more residential listings.
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 ensures that the regional distribution of numbers called is appropriate. Again, this works to increase the representativeness of the sample.
As many as 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 years of age or older, who is now at home.” If there is no eligible man at home, interviewers asked to speak with “the youngest female, 18 years of age or older, who is now 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 2006). 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.