The analysis in this report is based on telephone interviews conducted July 16-26, 2012 among a national sample of 2,508 adults, 18 years of age or older, living in all 50 U.S. states and the District of Columbia (1,505 respondents were interviewed on a landline telephone, and 1,003 were interviewed on a cell phone, including 531 who had no landline telephone). The survey was conducted by interviewers at Princeton Data Source and Universal Survey under the direction of Princeton Survey Research Associates International. Interviews were conducted in English and Spanish.
A combination of landline and cell phone random digit dial (RDD) samples were used; both samples were provided by Survey Sampling International. Both the landline and cell RDD samples were stratified by county based on estimated incidences of African-American and Hispanic adults, and counties with higher densities of African-American and Hispanic adults were oversampled. The final sample is weighted to correct for this disproportionate sampling. 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. For detailed information about our survey methodology, see https://www.pewresearch.org/politics/methodology/
The sample was divided into three racial/ethnic groups (Hispanics, non-Hispanic African Americans, and non-Hispanic whites/other race) for weighting; each group was weighted using an iterative technique that matches gender, age, education, nativity (among Hispanics) and region to parameters within each racial/ethnic group from the March 2011 Current Population Survey (CPS). The combined sample was then weighted to match gender, age, race, Hispanic origin and nativity, education and region to parameters from the March 2011 CPS and to match population density to a parameter from the Decennial Census. The sample also is 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 2011 National Health Interview Survey. The weighting procedure also accounts for the disproportionate sampling, adjusts for household size among respondents with a landline phone, and accounts for the fact that respondents with both a landline and cell phone have a greater probability of being included in the combined sample. 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.
Battleground states were identified using ratings for each state from late May to early June from: The Cook Political Report, MSNBC, The New York Times, Real Clear Politics, Karl Rove, CNN, Pollster.com, and the Washington Post. The ratings by these different groups yield 12 battleground states (rated as tossup or lean Republican or Democrat) and 39 safe states, including Washington, D.C. Battleground states are: Colorado, Florida, Iowa, Michigan, Missouri, Nevada, New Hampshire, North Carolina, Ohio, Pennsylvania, Virginia and Wisconsin. Solid or likely Republican states are: Alabama, Alaska, Arizona, Arkansas, Georgia, Idaho, Indiana, Kansas, Kentucky, Louisiana, Mississippi, Montana, Nebraska, North Dakota, Oklahoma, South Carolina, South Dakota, Tennessee, Texas, Utah, West Virginia and Wyoming. Solid or likely Democratic states are: California, Delaware, Washington D.C., Hawaii, Illinois, Maine, Maryland, Massachusetts, Minnesota, New Jersey, New Mexico, New York, Oregon, Rhode Island, Vermont and Washington.