The exit poll in 2010 was almost exactly correct, but what is it, and how does it actually work?

At 10pm on election day, the ballots will close, and the counting will begin. It may be many hours before it becomes clear which party has the most seats in the new parliament, and many days until we know which parties will form the next government. However, shortly after the 10 o’clock deadline, broadcasters will release a forecast based on the exit poll, which will give the first insight into how voters have actually voted in the general election. In this post, Jouni Kuha explains the detail behind the exit poll forecast, and indicates some of the difficulties in putting together a forecast under considerable time pressures