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Iowa: The Difference Between an Exit Poll and an Entrance Poll

An election exit poll is a survey of voters as they leave their voting location. In Iowa, we conduct an entrance poll of Iowans headed to their caucus locations. Exit polls typically ask voters about their attitudes on issues facing the country and what was important to their vote. They also ask about voters' demographics, such as age, gender, race and education. And of course, they ask for which candidate they voted.

The Iowa entrance poll collects all the same data, but there's a caveat: Those entering the caucus sites are asked whom they intend to support, not whom they just supported. Once they enter the caucus site, Democrats in Iowa may be persuaded to join another candidate's group, especially if their candidate does not reach a viability threshold.

Republicans actually do hold a vote, but it may be by show of hands, or after they have heard speeches supporting one candidate or another. On either side, Iowans may have a change of heart from what they just reported on an entrance poll to which candidate they actually end up supporting.

Additionally, those who arrive early at their caucus sites take the entrance surveys first. So the initial wave of data can look different than subsequent waves of data, filed after the caucus doors have closed at 8:00 pm EST.

In 2008, Hillary Clinton got her supporters out early and the first wave of data was very favorable to her. Later in the evening, after more interviews were completed, the entrance poll data showed Barack Obama would prevail, as more of his supporters arrived later. In 2012, Ron Paul backers showed up ahead of others, but at the end of the night, the polling data indicated that Rick Santorum and Mitt Romney were in a dead heat.

We here at NBC News will be crunching the entrance poll numbers through the night to provide careful analysis and help explain what's happening in Iowa.