Results for these surveys are based on telephone interviews conducted under the direction of Schulman, Ronca & Bucuvalas, Inc. among a nationwide sample of 2,000 adults, 18 years of age or older, a Chicago area sample of 801 adults, a Las Vegas area sample of 801 adults, a Phoenix area sample of 800 adults, a Raleigh-Durham area sample of 801 adults, and a Washington, DC area sample of 800 adults, each fielded during February 8 – March 7, 2006.
For results based on the total national sample, one can say with 95% confidence that the error attributable to sampling is plus or minus 2.5 percentage points. For national results based on form 1 (N=1000) and form 2 (N=1000), the sampling error is plus or minus 3.5 percentage points. For results based on a metropolitan area survey, the confidence interval is plus or minus 4 percentage points. For metropolitan area survey results based on form 1 and form 2 (N=400 approximately), the sampling error is plus or minus 5.5 percentage points.
Respondents who indicated they would prefer to complete the interview in Spanish, plus Spanish-speaking households in which no eligible English-speaking adult was available, were contacted by a Spanish-speaking interviewer. A total of 324 interviews were conducted in Spanish: National (69), Chicago (45), Las Vegas (73), Phoenix (70), Raleigh-Durham (36), and Washington, DC (31).
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 the national survey is a random digit sample of telephone numbers selected from telephone exchanges in the continental United States. The five metropolitan area surveys are random digit samples of telephone numbers selected from exchanges within the following Metropolitan Statistical Areas (MSA) as defined by the U.S. Census Department:
Chicago (Chicago-Naperville-Joliet, IL-IN-WI MSA)
IL: Cook, DeKalb, DuPage, Grundy, Kane, Kendall, Lake, McHenry, Will counties
IN: Jasper, Lake, Newton, Porter counties
WI: Kenosha county
Las Vegas (Las Vegas-Paradise, NV MSA)
NV: Clark county
Phoenix (Phoenix-Mesa-Scottsdale, AZ MSA)
AZ: Maricopa, Pinal counties
(Raleigh-Cary, NC MSA)
NC: Franklin, Johnston, Wake counties
(Durham, NC MSA)
NC: Chatham, Durham, Orange, Person counties
Washington, DC (Washington-Arlington-Alexandria, DC-VA-MD-WV MSA)
DC: District of Columbia
MD: Calvert, Charles, Frederick, Montgomery, Prince George’s counties
VA: Arlington, Clarke, Fairfax, Fauquier, Loudoun, Prince William, Spotsylvania, Stafford, Warren counties; Alexandria, Fairfax City, Falls Church, Fredericksburg, Manassas, Manassas Park independent cities
WV: Jefferson county
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. or within the MSA being surveyed. 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 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 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 for the national survey are derived from a special analysis of the most recently available Census Bureau’s Current Population Survey (March 2005). For the five metropolitan community surveys, data from Census 2000 was used. 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.