Results for this report are based on telephone interviews conducted from November 6-9, 2008 under the direction of Princeton Survey Research Associates International among a sample of 1,500 voters (“Voters” are those respondents who said they voted in the 2008 election.). The interviews were conducted among a population of 2,599 registered voters, 18 years of age or older, previously interviewed by Pew from October 16-19, 2008. Interviews were conducted on both landline telephones and cell phones (1,125 respondents were interviewed on a landline telephone, and 375 were interviewed on a cell phone, including 114 who had no landline telephone).
The combined landline and cell phone samples are weighted using an iterative technique that matches gender, age, education, race/ethnicity, region, and population density to parameters from the March 2007 Census Bureau’s Current Population Survey. The sample is also weighted to match current patterns of telephone status and relative usage of landline and cell phones (for those with both), based on extrapolations from the 2007 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 sample.
To improve comparability of the recontacted cases with the final 2008 election results, the data were also weighted to reflect the margin of the election results (an approximately 7-point Obama advantage). This step helps to minimize bias that could occur because certain types of respondents may have been easier or more difficult to re-interview.
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:
|Group||Sample Size||Plus or minus…|
|All voters||1,500||3.0 percentage points|
|Form 1 voters||744||4.0 percentage points|
|Form 2 voters||756||4.0 percentage points|
|Obama voters||767||4.0 percentage points|
|McCain voters||611||4.5 percentage points|
|Republican voters||438||5.5 percentage points|
|Democratic voters||548||5.0 percentage points|
|480||5.0 percentage points|
|Republican and Republican-leaning voters||611||4.5 percentage points|
|Democratic and Democratic-leaning voters||773||4.0 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.