When done right, exit polls are able to predict election results with a high degree of reliability.Unlike telephone opinion polls that ask people which candidate they intend to vote for should they vote, exit polls are surveys of voters conducted after they have cast their votes at their polling places. In other words, rather than a prediction of a hypothetical future action, they are a record of a just-completed action.
Around the world, exit polls have been used to verify the integrity of elections.The United States has funded exit polls in Eastern Europe in order to detect fraud. Discrepancies between exit polls and the official vote count have been used to successfully overturn election results in Ukraine, Serbia, and Georgia. Properly conducted, they remove nearly all of the sources of potential polling error. Unlike telephone polls, an exit poll will not be skewed by the fact that some groups of people tend not to be home in the evening or don’t own a landline telephone. Exit polls are not confounded by speculation about who will actually show up to vote, or by voters who decide to change their mind in the final moments. Rather, they identify the entire voting population in representative precincts and ask respondents immediately after voting to fill out a confidential questionnaire. Moreover, exit polls can obtain very large samples—well over 100,000 voter-respondents in the 2004 election, as compared to the typical telephone poll of 1,500 or so respondents—thus providing even greater degrees of reliability.
The difference between conducting a pre-election telephone poll and conducting an Election Day exit poll is like the difference between predicting snowfall in a region several days in advance of a snowstorm and estimating the region’s overall snowfall based on observed measures taken at representative sites. In the first case, you’re forced to predict future performance on present indicators, to rely on ambiguous historical data, and to make many assumptions about what may happen. In the latter, you simply need to choose your representative sites well. So long as your methodology is good and you read your measures correctly, your results will be highly accurate.
-- Freeman, Steven F., and Joel Bleifuss. Was the 2004 presidential election stolen?: exit polls, election fraud, and the official count. Seven Stories Press, 2011. pp 91-92