November 2008 exit poll methodology

About the 2009 general election exit polls in New Jersey and Virginia

Conducted for The Associated Press and other members of the National Election Pool (ABC, CBS, CNN, FOX and NBC) by Edison Research
Coverage: Representative surveys of voters in both New Jersey and Virginia. The surveys cover Election Day voters in person as they exit the polling place. The sample is designed so that everyone who votes in the general election on Election Day has a known probability of being included.
How many interviewed: About 2,000 voters statewide.
Exit polls: Stratified probability sample, taking into account population size and past voting history, of 40 precincts in each state.
- At each sampling location an interviewer approaches voters at a specified interval -- for example, every fifth voter -- as he or she exits the polling place. The interval helps ensure the randomness of the sample.
- Respondents check answers on a paper questionnaire prepared by the National Election Pool, including questions about demographics and issues related to the person's vote choices.
- Interviewing starts when the polls open and continues until about an hour before polls close.
- In places with large Hispanic populations, Spanish-language versions of the questionnaires are available.
- Participation is voluntary and anonymous.
Weighting: Survey results are adjusted to reflect the different probabilities of selecting a sample precinct and people attending each, as well as by the observed sex, race and estimated age of voters who refuse to participate or whom interviewers miss.
Sampling error: As with any survey, the results could vary because of chance variations in the sample. The larger the sample, the lower the sampling error. For example, in an exit poll of 1,000 voters, no more than one time in 20 should chance variations in the sample cause each result in the poll to vary by more than plus or minus 5 percentage points from the answers that would be obtained if voters in a given election were polled. For an exit poll of 1,500 voters the sampling error would be 4 points; for a survey of 2,000, 3 points.
- In exit polling, sampling error also depends on how many poll sites have voters with the characteristic of interest. For example, black or high-income voters may be found clustered only in certain sample precincts. Sampling error may be up to three times larger for clustered characteristics.
Polls are subject to other sources of error, such as from question wording or order.

Additional information: Edison Research site