Results for this survey are based on telephone interviews conducted under the direction of Princeton Survey Research Associates International among a nationwide sample of 2,007 adults, 18 years of age or older, from October 17-23, 2007 (1,507 respondents were interviewed on a landline telephone, and 500 were interviewed on a cell phone, including 199 who had no landline telephone). Both the landline and cell phone samples were designed by Survey Sampling, Inc.
The combined landline and cell phone data were weighted using demographic weighting parameters derived from the March 2006 Census Bureau’s Current Population Survey, along with an estimate of current patterns of telephone status in the U.S. derived from the 2006 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 Pew Research Center for the People & the Press will be releasing a detailed analysis of the landline and cell phone samples in the coming weeks.
For results based on the total sample, one can say with 95% confidence that the error attributable to sampling is plus or minus 2.5 percentage points. For results based on Form 1 (N=1,008) or Form 2 (N=999), one can say with 95% confidence that the error attributable to sampling is plus or minus 3.5 percentage points. For results based on Democratic and Democratic-leaning registered voters (N=837) the margin of error is plus or minus 4 percentage points. For Republican and Republican-leaning registered voters (N=648) the margin of error is plus or minus 4.5 percentage points.
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.