History shows that even in an normal year, Iowans rarely settle on a final decision until the last possible moment. In past election cycles, the final four-day poll conducted just before the caucuses shows how much turbulence remains late in the race.

Perhaps the most dramatic four-day poll was in 2012. Day One had Ron Paul with a 7-point lead over Mitt Romney, followed by Rick Perry and Newt Gingrich tied for third, with Rick Santorum trailing them by 2 points. By the final day, Santorum had surged to within a point of Romney, and Paul had fallen to third place. Santorum’s trajectory was so striking, it became the lead finding from the poll, even though he was still 1 point behind Romney. We all remember what happened next: A vote on caucus night that should have been too close to call. But it was called a win for Romney—by eight votes. The verification procedure that followed overturned that call, with Santorum declared the winner by 34 votes.

In 2008, the opening day of interviewing showed Barack Obama one point ahead of Hillary Clinton, with John Edwards trailing by another 5 points. Just three days later, on the final day of the poll, Obama enjoyed a 6-point lead over Edwards, with Clinton falling to third. On the Republican side that year, Romney had a second-day spike over Mike Huckabee, but on the final day, it was Huckabee by 4 points—far narrower than the 11-point advantage he had enjoyed on the first day of interviewing.