What is an election exit poll and how accurate is it?

Labour has secured a sweeping victory in the general election, just as the exit poll predicted.

Labour Party leader Keir Starmer and wife Victoria arrive at a polling station to cast their vote in London, Thursday, July 4, 2024. Voters in the U.K. are casting their ballots in a national election to choose the 650 lawmakers who will sit in Parliament for the next five years. Outgoing Prime Minister Rishi Sunak surprised his own party on May 22 when he called the election. (AP Photo/Vadim Ghirda)
Keir Starmer and his wife Victoria voting on Thursday. (AP)

Labour has secured a sweeping victory in the general election, just as the exit poll predicted.

According to the forecast carried out by pollsters Ipsos, Labour was on course to win a huge majority in the House of Commons and end up with 410 seats to the Conservative Party’s 131, with the Lib Dems on 61 and Reform UK on 13, the SNP on 10, Plaid Cymru on 4 and the Greens on 2

Labour was projected to secure a majority of 170 seats in this election, aligning closely with its 1997 election success of 179 seats.

Keir Starmer's party is expected to attain a similar number in this election, currently holding 174 seats with just two constituencies left to declare.

With 646 constituency results declared, Labour emerged victorious in 412 seats. The Conservatives secured 121 seats, while the Liberal Democrats triumphed in 71 constituencies.

Reform UK managed to secure four seats.

The SNP has nine seats, Plaid Cymru secured four, Sinn Fein has seven, and the DUP has five. The Green Party gained 6.8% of the votes and four seats.

The result follows weeks of projections suggesting Rishi Sunak's Conservative Party is set to be wiped out across swathes of the country.

Exit polls have been used internationally since at least the late 1960s, with pollsters asking voters their intention as they leave the polling booth.

The exit poll in the UK covers England, Scotland and Wales (in Northern Ireland, voters choose from a different set of parties).

Polling companies use teams of statistics experts to make sense of the data from exit polls in more than 100 constituencies and thousands of voters.

Pollsters visit a selected polling station in a constituency and ask selected voters to fill in a replica ballot paper after they have filled in their real one.

The idea of this is that people tend to be more honest while writing down their vote than saying it out loud.

The experts compare data on the day to previous elections, looking for changes in voting behaviour.

EDINBURGH, SCOTLAND - JUNE 24: Professor Sir John Curtice after speaking at a conference of the Law Society of Scotland marking 20 years of devolution and the Scottish Parliament, on June 24, 2019 in Edinburgh, Scotland. (Photo by Ken Jack/Getty Images)
Professor Sir John Curtice leads Britain's team of exit pollsters. (Getty Images)

The experts take the small samples from polling stations and analyse this to find broader geographic trends in how people are voting - for example, whether there is a swing towards Labour in the north of England.

Statistical analysis allows experts – teams of highly respected academic statisticians led by pollster Professor Sir John Curtice – to come up with a projected figure of how many seats each party will get.

Generally speaking, exit polls in the UK have become more accurate over time, as the techniques used have improved.

Exit polls offer an often accurate depiction of what might happen in the real election. For example, in 2019, the exit poll forecast 368 for Conservatives and 191 for Labour, and the real figure was 365 and 203 seats.

Professor Curtice, who leads Britain’s exit polling team, says that the results offer a "pretty good indication" of the real result.

“The record isn’t perfect, but since 2005 the exit poll has given a pretty good indication of where the result will end up on the night," he told the Telegraph. "It also tends to be more accurate than opinion polls carried out before voting happens.”

Veteran broadcaster David Dimbleby, however, told the BBC's Newscast podcast he was never a fan of exit polls because they take the "excitement" out of election night.

He said: “The exit poll is the bane of the broadcaster’s life. It’s the worst invention ever brought in – it’s like a thriller and you’re given the answer before we’ve even started on page one.

“It gives people something to talk about until three in the morning when the first serious results flow starts. But I never liked them. It takes the fun away.”

Television presenter and broadcaster David Dimbleby speaks to pro-remain protesters outside The Supreme Court as the first day of the hearing to rule on the legality of suspending or proroguing Parliament begins on September 17th 2019 in London, United Kingdom. The ruling will be made by 11 judges in the coming days to determine if the action of Prime Minister Boris Johnson to suspend parliament and his advice to do so given to the Queen was unlawful. David Dimbleby is a British journalist and former presenter of current affairs and political programmes, now best known for the BBC's long-running topical debate programme Question Time. (photo by Mike Kemp/In Pictures via Getty Images)
Veteran BBC broadcaster David Dimbleby has confessed he was never keen on the idea of exit polls. (Getty Images)

But with Britain’s political landscape shifting, polls can sometimes fail - in 2015, the exit poll predicted a hung parliament rather than a Conservative majority.

This year’s exit poll will take into account data including how constituencies voted in the 2016 referendum.

Also, in 1992, exit polls for the BBC forecast a hung parliament, but the Conservatives won 65 more seats than Labour with a majority of 21 seats.

Dimbleby said this was an example where the exit polls had "completely screwed up", although the statistical techniques used in exit polling have evolved significantly since then.

If the exit poll turns out to be accurate, the projection of seats for Labour would give it a similar majority to when Tony Blair was in power.

It predicts Starmer will almost equal 418 seats won by Blair in Labour's landslide in 1997, when it secured a majority of 179 seats.

When Blair was re-elected with another landslide four years later in 2001, becoming the only Labour prime minister to serve two consecutive full terms in office, his party returned 412 MPs and ended up with a majority of 167 seats.