The analysis in this report is based on telephone interviews conducted June 15-26, 2016 among a national sample of 2,245 adults, 18 years of age or older, living in all 50 U.S. states and the District of Columbia (559 respondents were interviewed on a landline telephone, and 1,686 were interviewed on a cell phone, including 1,067 who had no landline telephone). The survey was conducted under the direction of Abt SRBI. Interviews were conducted in English and Spanish. For detailed information about our survey methodology, see https://www.pewresearch.org/methodology/u-s-survey-research/
A combination of landline and cell phone random digit dial samples were used; samples were provided by Survey Sampling International.
The first sample was a random-digit dialing (RDD) landline sample; a total of 500 interviews were completed using this RDD landline sample. The second sample was a RDD cell sample; a total of 1,500 interviews were completed using this RDD cell sample. 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.
Additional samples from both the landline and cellular RDD frames were drawn to achieve an oversample of Hispanics. The selection of these oversamples was similar to the other RDD samples, with the exception that respondents were screened to determine if they were of Hispanic, Latino or Spanish origin; if not, then the interview was terminated. Hispanic respondents in the landline sample were selected by randomly asking for the youngest Hispanic adult male or female now at home. A total of 543 Hispanic respondents were interviewed, 245 in the oversample (59 were interviewed on a landline telephone, and 186 were interviewed on a cell phone), and 298 in the main RDD sample (45 were interviewed on a landline telephone and 253 were interviewed on a cellphone).
The combined landline and cell phone samples are weighted using an iterative technique that matches gender, age, education, race, Hispanic origin and nativity and region to parameters from the 2014 Census Bureau’s American Community Survey and population density to parameters from the Decennial Census. The weighting procedure accounts for the additional interviews with Hispanic respondents. 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 2015 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 interviewed on a landline phone (Hispanic household size among the Hispanic oversample landline respondents). 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 sizes and the error attributable to sampling that would be expected at the 95% level of confidence for different groups in the survey:
Sample sizes and sampling errors for other 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.
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