The analysis in this report is based on telephone interviews conducted May 2, 2011 among a national sample of 654 adults 18 years of age or older living in the United States (555 respondents were interviewed on a landline telephone, and 99 were interviewed on a cell phone and had no landline telephone). The survey was conducted by interviewers at Abt/SRBI Inc. A combination of landline and cell phone random digit dial samples were used; both samples were provided by Survey Sampling International. Interviews were conducted in English. 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, and they did not have a landline telephone.
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 2009 American Community Survey. The sample also is weighted to match current patterns of telephone status, based on estimates for adults with telephones from the January-June 2010 National Health Interview Survey. 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 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. Surveys conducted in a single day are potentially subject to additional error, including the fact that people who are infrequently at home may have a smaller chance of being included in the poll than in surveys that span multiple days.