Most of the analysis in the 2011 Typology Report is based on telephone interviews conducted February 22 – March 1, 2011 and March 8-14, 2011 among a national sample of 3,029 adults 18 years of age or older living in the continental United States (2,026 respondents were interviewed on a landline telephone, and 1,003 were interviewed on a cell phone, including 455 who had no landline telephone). In addition to the main survey, some results are based on a callback survey where an attempt was made to re-interview respondents to the original survey. This short follow-up survey, conducted April 7-10, 2011 obtained interviews with 1,432 respondents from the original typology survey.
The surveys were conducted by interviewers at Princeton Data Source under the direction of Princeton Survey Research Associates International. A combination of landline and cell phone random digit dial samples were used; both samples were provided by Survey Sampling International. Interviews for the main survey were conducted in English and Spanish. Interviews for the callback survey were conducted in English only. Respondents in the landline sample were selected by randomly asking for the youngest adult male or female who is currently at home. Interviews in the cell sample were conducted with the person who answered the phone, if that person was an adult 18 years of age or older.
The combined landline and cell phone sample for each survey is weighted using an iterative technique that matches gender, age, education, race, Hispanic origin, region and population density to parameters from the March 2010 U.S. Census Bureau’s Current Population Survey. The samples also are weighted to match current patterns of telephone status and relative usage of landline and cell phones (for those with both), based on extrapolations from the 2010 National Health Interview Survey. The weighting procedure also accounts for the fact that respondents with both landline and cell phones have a greater probability of being included in the combined sample and adjusts for household size within the landline sample. Sampling errors and statistical tests of significance take into account the effect of weighting.
The following table shows the sample sizes and the error attributable to sampling that would be expected at the 95% level of confidence for the total sample and for the typology groups in each survey:
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 typical Pew Research Center for the People & the Press national survey selects a random digit sample of both landline and cell phone numbers in the continental United States. As the proportion of Americans who rely solely or mostly on cell phones for their telephone service continues to grow, sampling both landline and cell phone numbers helps to ensure that our surveys represent all adults who have access to either (only about 2% of households in the U.S. do not have access to any phone). We sample landline and cell phone numbers to yield a ratio of approximately two completed landline interviews to each cell phone interview. This ratio is based on an analysis that attempts to balance cost and fieldwork considerations as well as to improve the overall demographic composition of the sample (in terms of age, race/ethnicity and education). This ratio also ensures an adequate number of cell-only respondents in each survey.
The design of the landline sample ensures representation of both listed and unlisted numbers (including those not yet listed) by using random digit dialing. This method uses random generation of the last two digits of telephone numbers selected on the basis of the area code, telephone exchange, and bank number. A bank is defined as 100 contiguous telephone numbers, for example 800-555-1200 to 800-555-1299. The telephone exchanges are selected to be proportionally stratified by county and by telephone exchange within the 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 banks of telephone numbers containing three or more listed residential numbers are selected.
The cell phone sample is drawn through systematic sampling from dedicated wireless banks of 100 contiguous numbers and shared service banks with no directory-listed landline numbers (to ensure that the cell phone sample does not include banks that are also included in the landline sample). The sample is designed to be representative both geographically and by large and small wireless carriers.
Both the landline and cell samples are released for interviewing in replicates, which are small random samples of each larger sample. Using replicates to control the release of telephone numbers ensures that the complete call procedures are followed for all numbers dialed. The use of replicates also improves the overall representativeness of the survey by helping to ensure that the regional distribution of numbers called is appropriate.
When interviewers reach someone on a landline phone, they randomly ask half the sample if they could speak with “the youngest male, 18 years of age or older, who is now at home” and the other half of the sample to speak with the “youngest female, 18 years of age or older, who is now at home.” If there is no eligible person of the requested gender at home, interviewers ask to speak with the youngest adult of the opposite gender, who is now at home. This method of selecting respondents within each household improves participation among young people who are often more difficult to interview than older people because of their lifestyles.
Unlike a landline phone, a cell phone is assumed in Pew Research polls to be a personal device. Interviewers ask if the person who answers the cell phone is 18 years of age or older to determine if the person is eligible to complete the survey. This means that, for those in the cell sample, no effort is made to give other household members a chance to be interviewed. Although some people share cell phones, it is still uncertain whether the benefits of sampling among the users of a shared cell phone outweigh the disadvantages.
Sampling error results from collecting data from some, rather than all, members of the population. For each of our surveys, we report a margin of sampling error for the total sample and often for key subgroups analyzed in the report. For example, the sampling error for a typical national survey of 1,500 completed interviews is plus or minus 3 percentage points with a 95% confidence interval. This means that in 95 out of every 100 samples of the same size and type, the results we obtain would vary by no more than plus or minus 3 percentage points from the result we would get if we could interview every member of the population. Thus, the chances are very high (95 out of 100) that any sample we draw will be within 3 points of the true population value. The sampling errors we report also take into account the effect of weighting.
At least 7 attempts are made to complete an interview at every sampled telephone number. The calls are staggered over times of day and days of the week (including at least one daytime call) to maximize the chances of making contact with a potential respondent. Interviewing is also spread as evenly as possible across the field period. An effort is made to recontact most interview breakoffs and refusals to attempt to convert them to completed interviews.
Response rates for Pew Research polls typically range from 5% to 20%; these response rates are comparable to those for other major opinion polls. The response rate is the percentage of known or assumed residential households for which a completed interview was obtained. The response rate we report is computed using the American Association for Public Opinion Research’s Response Rate 3 (RR3) method. Fortunately, low response rates are not necessarily an indication of nonresponse bias. Nonresponse in telephone interview surveys can produce biases in survey-derived estimates. Survey participation tends to vary for different subgroups of the population, and these subgroups are likely to also vary on questions of substantive interest. To compensate for these known biases, the sample data are weighted for analysis.
The landline sample is first weighted by household size to account for the fact that people in larger households have a lower probability of being selected. In addition, the combined landline and cell phone sample is weighted to account for the fact that respondents with both a landline and cell phone have a greater probability of being included in the sample. The sample is then weighted using population parameters from the U.S. Census Bureau for adults 18 years of age or older living in the continental United States. The parameters for age, education, race/ethnicity, and region are from the Current Population Survey’s March 2010 Annual Social and Economic Supplement and the parameter for population density is from the Decennial Census. These population parameters are compared with the sample characteristics to construct the weights. In addition to the demographic parameters, the sample is also weighted to match current patterns of telephone status and relative usage of landline and cell phones (for those with both), based on extrapolations from the 2010 National Health Interview Survey. The final weights are derived using an iterative technique that simultaneously balances the distributions of all weighting parameters.
Weighting cannot eliminate every source of nonresponse bias. Nonetheless, properly-conducted public opinion polls have a good record in achieving unbiased samples. In particular, election polling – where a comparison of the polls with the actual election results provides an opportunity to validate the survey results – has been very accurate over the years.
For more information about our survey methodology, see https://legacy.pewresearch.org/politics/methodology/.
