Editor’s note: This story was updated after polls closed at 2200GMT

FOR THE third time in less than five years Britain has voted to elect 650 parliamentarians. Polling stations have closed but the first results will not be expected until after 11pm. An exit poll at 10pm, when polling stations closed, predicts that that the Conservatives, led by the current prime minister, Boris Johnson, will win an overwhelming majority. How accurate is the exit poll?

On election day in 1970 the BBC tried something “entirely new”: an exit poll at Gravesend in Kent, which was “shown by a computer to be the most ordinary constituency in England”. It correctly anticipated a surprise victory for the Conservative Party. Nearly half a century later, a nationwide exit poll has become an essential element of British election-night coverage.

It is perhaps the world’s most expensive one-question social survey. Britain’s three main broadcasters—BBC, ITV and Sky News—jointly commission the exit poll from Ipsos Mori, a pollster, and a team of academics, at a cost of some £300,000 ($395,000). The work takes place in two stages. First, pairs of fieldworkers are dispatched to 144 of Britain’s 40,000 polling stations between the hours of 7am, when voting began, until it ended. At each location they ask approximately every eighth person leaving to fill in a dummy polling card containing a single question, “How did you vote?”, and deposit it anonymously into a cardboard box.

With no other information gathered about the 30,000 respondents to help make the projection (as happens in American exit polls, for instance), the accuracy of the exit poll rests upon surveying the same locations in each general election. These were chosen in 2001 to be broadly representative of Britain, but also to be telling enough in the event of a tight national race between the Conservatives and the Labour Party. The academics are not concerned with the level of support for parties in the selected places, but the change compared with the previous exit poll (there are no actual results for individual polling stations).

The second stage involves the academics working throughout the day to create models that explain why support for the different parties has changed. These are then applied to each of the 632 constituencies in Britain—the 18 seats in Northern Ireland are treated differently—to arrive at seat-level estimates of vote share and the probability that any given party will be victorious. Adding up the probabilities for each party in each seat produces a forecast for a party’s expected number of seats nationwide.

Like any statistical exercise, the exit poll comes with a degree of uncertainty. David Firth, of the University of Warwick, who helped pioneer the current technique, reckons that any forecast for the largest party’s seat total that is within ten seats of its final tally should be considered as “exceptionally accurate”. Remarkably, in all but one of the past five exit polls, in 2015, that error has been five seats or fewer (see chart).

This year’s exit poll has been a more difficult undertaking, for three reasons. First, the anti-EU Brexit Party was standing for the first time, but only in seats that were not already held by the Tories. Although support for the party fell in recent weeks to just 2% nationally, according to polls, modelling how they might affect the ability of the Tories to win Labour-held marginal seats caused a headache for the academics. Second, tactical voting, particularly among those wanting to remain in the EU, might have been more widespread in this election, making the modelling trickier.

The greatest source of uncertainty came from the postman: Britain’s first December election since 1923—elections are normally held in the spring—may have produced a higher share of postal votes. If the composition of postal voters is different from 2017, when 21.6% of voters submitted their ballots by mail, that could cause greater uncertainty. Although the academics’ central estimate is that the Tories will win a healthy 86-seat majority, the poll does come with a forecast error of about 15 seats either side.

As results are declared throughout the night The Economist will be producing a live forecast model that will project the parties’ probable final seat tally.

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