Why did the election pollsters get it so wrong?

The British Polling Council (BPC) launched the inquiry following confirmation that David Cameron’s party had secured a Commons majority, despite a run of polls showing the Tories and Labour neck and neck and a hung parliament the most likely outcome.

The accuracy of pre-election polls was called into question at 10 o’clock on Thursday night when the broadcasters’ exit poll, based on which parties people had voted for, showed the Conservatives way ahead of Labour.

‘Eat my hat’

Former Liberal Democrat leader Sir Paddy Ashdown was so shocked by the exit poll, which forecast that his party would end up with just 10 seats, that he told the BBC he would “eat my hat” if it was borne out by the results. In fact, the Lib Dems finished with eight seats.

Throughout the campaign, UK-wide polls suggested that neither Labour nor the Tories had made a breakthrough and there would not be a clear winner, although they envisaged that the Conservatives would win more seats than Labour. They had more success in Scotland, where an SNP wipeout in Labour’s former heartland was accurately forecast.

Martin Boon of ICM speaking for the entire industry here https://t.co/jLIpJxOFwb

— Adam Drummond (@AGKD123) May 8, 2015

A terrible night for us pollsters. I apologise for a poor performance. We need to find out why.

— Stephan Shakespeare (@StephanShaxper) May 8, 2015

Election results raise serious issues for all pollsters. We will look at our methods & hv urged the Brtsh Polling Council to set up a review

— Populus (@PopulusPolls) May 8, 2015

This failure was neatly summed up by ICM’s Martin Boon, who tweeted “oh shit” after the publication of the exit poll. Chancellor George Osborne said the pollsters faced “a big post-mortem”.

The BPC, which counts all major UK pollsters among its members, said in a statement: “The final opinion polls before the election were clearly not as accurate as we would like, and the fact that all the pollsters underestimated the Conservative lead over Labour suggests that the methods that were used should be subject to careful, independent investigation.”

It said it was “setting up an independent inquiry to look into the possible causes of this apparent bias and to make recommendations for future polling”.

YouGov President Peter Kellner told the BBC: “What seems to have gone wrong is that people have said one thing and they did something else in the ballot box. We are not as far out as we were in 1992, not that that is a great commendation.”

‘Shy Tories’

In 1992, the pollsters also under-estimated John Major’s Conservatives, who won the election with more votes than any party has ever managed. At the time, the disparity was attributed to “shy Tories” who do not want to admit to pollsters that they are voting Conservative.

Michelle Harrison, from TNS, told Sky News it had been a “mixed night for the polling community”. While it had correctly forecast the result in Scotland and the fall in the Lib Dem vote, “the distribution of the seats may not be our greatest moment”.

Even the exit poll under-estimated the number of seats the Tories would win – 316 versus the 331 they ended up with. But it was closer with Labour, forecasting 239 seats, against the 232 the party won.