Methodology

This report is based on telephone interviews conducted October 6-10, 2017 among a national sample of 1,012 adults, 18 years of age or older, living in all 50 U.S. states (406 respondents were interviewed on a landline telephone, and 606 were interviewed on a cell phone, including 378 who had no landline telephone). The survey was conducted under the direction of SSRS. A combination of landline and cell phone random digit dial samples were used. Interviews were conducted in English and Spanish. Respondents in the landline sample were selected by randomly asking for the youngest adult male or female who is now 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. For detailed information about our survey methodology, see www.pewresearch.org/methodology/u-s-survey-research/.

The combined landline and cell phone sample are weighted using an iterative technique that matches gender, age, education, race, Hispanic origin and region to parameters from the March 2017 supplement of the U.S. Census Bureau’s Current Population Survey and county-level population density to parameters from the 2010 Decennial Census. The sample also is weighted to match current patterns of telephone status (landline only, cell phone only, or both landline and cell phone), based on extrapolations from the 2016 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 among respondents with a landline phone. The margins of error reported and statistical tests of significance are adjusted to account for the survey’s design effect, a measure of how much efficiency is lost from the weighting procedures.

The following table shows the unweighted sample size and the error attributable to sampling that would be expected at the 95% level of confidence for the total sample:

Sample sizes and sampling errors for subgroups are available upon request.

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.