Exit polls: How accurate are they? The science of predicting the outcome of an election

30 June 2024, 23:04 | Updated: 1 July 2024, 10:41

It is the first big moment of election night. The exit poll is the moment millions tune in for a first sniff of the eventual result of the general election. 

And in Election 2024 this poll, with its impressive track record, sometimes down to a margin of only a few seats, will, once again, be a key part of broadcasters' coverage - including here at Sky News - on Thursday night.

The current model was devised in 2005 by Professor John Curtice and statistician David Firth and it has been consistently reliable, bar 2015 when the seat numbers suggested a hung parliament and David Cameron scraped a thin majority.

But for the most part, its accuracy has been dependable. In 2010, it correctly predicted the exact number of seats for the Conservatives.

Commissioned by the broadcasters - Sky News, the BBC and ITV News - the fieldwork is carried out by IPSOS UK who will have interviewers at 133 polling stations around the country this year.

People who have just voted will be asked to privately fill in a replica ballot paper and place it into a ballot box as they leave their local community centre, church hall or station.

Michael Clemence, from IPSOS UK, says this and the scale are part of what distinguishes the exit poll from the many surveys that have come before it.

"We're going to be doing over 17,000 interviews on the day. And also we're dealing with people's behaviour. So we're not asking people how they intend to vote.

"We're talking to electors who just voted. And I'm asking exactly what they just did. So you've cut out the error in prediction polling."

Researchers can only deploy to a fraction of the total constituencies in England, Scotland and Wales, so locations are chosen to best reflect the demographics of the country with an urban and rural spread.

However, many of the locations will be in marginal seats, where the swing between the main parties will be tracked.

The same polling stations are targeted year after year so the swing from the last election's exit poll, along with other data at constituency level, can be analysed by those crunching the numbers.