Results for this survey are based on telephone interviews conducted under the direction of Princeton Survey Research Associates International among a national sample of 1,007 adults living in the continental United States, 18 years of age or older, from July 1-5, 2010 (675 respondents were interviewed on a landline telephone, and 332 were interviewed on a cell phone, including 136 who had no landline telephone). Interviews were not conducted on Independence Day, July 4. Both the landline and cell phone samples were provided by Survey Sampling International. Interviews were conducted in English.
The combined landline and cell phone sample are weighted using an iterative technique that matches gender, age, education, race, Hispanic origin, region, and population density to parameters from the March 2009 Census Bureau’s Current Population Survey. The sample is also weighted to match current patterns of telephone status, based on extrapolations from the 2009 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 error attributable to sampling that would be expected at the 95% level of confidence for different groups in the 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.