On election night, when can you start to know who’s won?

In Britain for the past two decades, there’s been a startlingly good answer just after the polls close, at 10 p.m.

That’s when three major broadcasters reveal the results of the national exit poll. The work of a team of statisticians and political scientists who swing into gear a few hours earlier, it has in recent years produced an increasingly accurate picture of the election results before the votes have been counted.

In the past five British general elections, the exit poll has predicted how many of the 650 or more parliamentary seats would be claimed by the winning party to within an average of four seats. Last time, in 2019, it had the winning party’s total just three seats out.

Here’s a guide to what to expect, and how it works.

Let’s start at the beginning. What’s an exit poll?

It’s a survey of voters soon after they’ve voted. The British one looks for voters literally as they exit a polling place: Fieldworkers ask over 20,000 people at about 130 voting sites across the country to fill in replica ballot papers. Since 2005, there’s been a single exit poll at each British general election, paid for by three major broadcasters, the BBC, ITV and Sky.

How have the British ones been so accurate?

They weren’t always. In the 1992 general election, the BBC’s exit poll predicted that no party would win an overall majority of parliamentary seats, before early results quickly showed that the Conservatives were on course to retain control. Exit polls in some earlier elections were even further off.