Exit poll methodology -- Super Tuesday primaries
Voter surveys in the Mar. 6, 2012, presidential primaries were conducted for the National Election Pool (The Associated Press, ABC, CBS, CNN, FOX and NBC) by Edison Research. NEP members prepared the questionnaires.
Election day surveys were conducted in the following number of precincts per state:
The polling places are a stratified probability sample of each state, factoring in size and past voting history. The sample is designed so that everyone who voted in the state's primaries Tuesday has a known probability of being included.
At each sampling location an interviewer approached voters at a specified interval -- for example, every fifth voter -- as he or she exited the polling place. The interval helps ensure the randomness of the sample. In states where both Democrats and Republicans held primaries, voters were asked which primary they had just voted in and were given a paper questionnaire specific to that primary.
In Ohio and Tennessee -- states with expected high incidence of absentee or early voting -- telephone polls also were conducted from Feb. 27 - Mar. 4, screening for people who had already voted or said they were certain to vote before Tuesday. Results from the telephone interviews were blended into the election day samples, weighted to the estimated proportion of early voting among the overall electorate. These surveys included 410 voters in Ohio and 640 in Tennessee.
All samples are approximations. A measure of the approximation is called the sampling error. Sampling error is affected by the design of the sample, the characteristic being measured and the number of people who have the characteristic. If a characteristic is found in roughly the same proportions in all precincts ("non-clustered") the sampling error will be lower. If the characteristic is concentrated in a few precincts the sampling error will be larger. Gender would be a good example of a characteristic with a lower sampling error. Characteristics for minority racial groups will have larger sampling errors.
For these polls, the margin of sampling error for results based on the full sample was plus or minus 4 percentage points in Georgia, Massachusetts, Oklahoma, Vermont and Virginia, and plus or minus 3 percentage points in Ohio and Tennessee.
The table below lists typical sampling errors for this exit poll for non-clustered characteristics at the 95 percent confidence interval. No more than one time in 20 should chance variations in the sample cause the results to vary by more than this amount for non-clustered characteristics. However, non-sampling factors such as question wording and order and voter non-response may increase total error.