Results for this survey are based on telephone interviews conducted under the direction of Abt SRBI, Inc., among a nationwide sample of 2,905 adults, 18 years of age or older, from July 31-August 10, 2008 (2,254 respondents were interviewed on a landline telephone, and 651 were interviewed on a cell phone, including 262 who had no landline telephone). Both the landline and cell phone samples were provided by Survey Sampling International.
The combined landline and cell phone data were weighted using demographic weighting parameters derived from the March 2007 Census Bureau’s Current Population Survey, along with an estimate of current patterns of telephone status in the U.S. derived from the 2007 National Health Interview Survey, using an iterative technique that simultaneously balances the distributions of all weighting parameters. The weighting procedure also accounted for the fact that respondents with both landline and cell phones had a greater probability of being included in the sample.
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.