Get ready to ignore those early exit-poll numbers.
Exit-poll numbers are useful tools for news media organizations that wish to call states for a candidate before all votes have been counted. They also are useful for gauging turnout rates and voting preferences for various demographic groups. But incomplete exit-poll data leaked during Election Day, while the polls remain open, can be woefully misleading. Pollsters have taken steps to prevent leaks, but it’s worth keeping this in mind in case you do see early numbers.
In 2004, leaked exit-poll numbers suggested a positive outlook for Sen. John Kerry, including leads in Florida and Ohio, that led some election watchers astray. Early exit-poll numbers, possibly fabricated, were also fixtures of the primaries this year, and they often missed the mark on the final outcome, generally leaning too heavily toward Sen. Barack Obama.
Even final exit-poll numbers are subject to greater sampling error than telephone polls, in part because there are two stages of random selection: first, of precincts in which to poll, and second, of voters at those precincts. Also, because they are conducted in person, interviewers may have a bigger effect on respondents. The 2004 exit polls showed Kerry winning by 5.5 percentage points nationally and overstated his vote share by more than the margin of error in 26 states, according to a post-election analysis by the exit pollsters. Younger interviewers had bigger errors toward Kerry.
Exit pollsters took steps to address those problems, but we won’t really know how well they did until the votes are counted.
Further reading: Pollster.com blogger Mark Blumenthal has answered questions you didn’t even think to ask about exit polls in his ongoing series (including “Why were exit polls in Germany & Utah so accurate?“). Nate Silver, who provides election forecasts at fivethirtyeight.com, offers 10 reasons for ignoring exit polls entirely. Time provides a brief history of exit polling in the U.S.