One of the most difficult aspects of conducting election polls is determining whether a respondent will actually vote in the election. More respondents say they intend to vote than actually will cast a ballot. As a consequence, pollsters do not rely solely upon a respondent’s stated intention when classifying a person as likely to vote or not. Most pollsters use a combination of questions that measure intention to vote, interest in the campaign and past voting behavior. Different pollsters use different sets of questions to help identify likely voters.

The Pew Research Center’s likely voter questions from 2008 can be found in Pew Research Election Questions in red type. We use nine questions to assign each respondent a score on the likely voter scale in our final pre-election poll. Earlier in the campaign, we use a somewhat shorter version of the scale to identify likely voters. For more extensive detail about likely voters in the 2008 presidential election and the scale used to determine who was most likely to vote see Understanding Likely Voters.

The Pew Research Center’s likely voter scale is very similar to the method developed by election polling pioneer Paul Perry at the Gallup Organization and has been used successfully in many elections. To help assess which questions or combination of questions from the scale were most accurate in predicting voting, we conducted an experiment in a closely contested mayoral race in Philadelphia in 1999. The results from the experiment are summarized in Screening Likely Voters: A Survey Experiment and a more extensive discussion of how the likely voter scale is created and used is available in Screening for Likely Voters in Pre-Election Polls: A Voter Validation Experiment.

These publications contain the results of the Pew Research Center’s final presidential election forecast polls: