Originally Posted by super-niall
WTF is an exit poll?
Interviewers stop every nth person coming out the polling station and give them a mock ballot paper to fill in, if someone refuses they are not replaced by another person. Every hour, the papers are collected and phoned back to HQ, where they are weighted for differential response rates.
The first result comes out at 10pm on the dot, with a final projection at 11pm or so.
The aim of the exit poll is to predict the seat totals, not the share of the vote, and the team will try to work out if there are different shifts in support in different types of seat. The call is based on a probability of each seat going one way or the other, all summed up to make a seat total.
In terms of past accuracy, the exit poll last time got the Labour majority exactly right (though they were slightly off with Conservative and Lib Dem seats). Unless something goes terribly wrong, we should have a broad idea of the result a couple of minutes after 10 o'clock.