All of the pollsters vastly underestimated David Cameron’s party and were strikingly consistent in how wrong they were

“Well hung” was the headline on the Sun’s front page, while the Guardian ran with “It couldn’t be closer”. The polls were predicting Labour and the Conservatives were neck-and-neck, but they couldn’t have been more wrong.

All of the polls significantly underestimated the Tories, and they were strikingly consistent in how wrong they were. Populus put Labour one point ahead, BMG, Survation, YouGov and polling by the Conservative peer Lord Ashcroft all predicted a tie. Opinium predicted a one-point lead for the Tories, as did ComRes and Ipsos Mori.

So when the exit polls just after 10pm predicted 316 seats for the Tories – up nine seats from 2010 – and Labour a devastating 239 seats, with the Lib Dems obliterated to just 10 and the SNP winning all but one seat in Scotland, many could not believe it was true.

Lib Dem grandee Paddy Ashdown promised to eat his hat on live TV if the exit polls were correct.

But although the figures were not 100%, they were close to the mark. Pollsters had vastly underestimated David Cameron’s party.

YouGov’s poll on Thursday night, conducted after votes had been cast, was baffling. It again showed that the parties were neck-and-neck.

Some analysts have speculated that polling companies may have been “herding”, skewing their polls towards an average, a theory boosted by how closely polls aligned in the final days.

Harry Enten (@ForecasterEnten) To me there should be a wider spread in the final numbers from a statistical sense... We'll see if the pollsters have converged correctly.

Late election polls are notoriously hit-and-miss. No pollster predicted the Labour or Lib Dem share of the vote correctly in 2010. In 1992, the closest comparable election to 2015 in its character, pollsters without exception vastly underestimated the Tories. The Tory lead was 7.6 points, the closest was Gallup, which predicted 0.5 point.

The same excuses are being floated this time as were used in 1992: changing methods of polling, “shy Tories” who did not want to admit they were voting for John Major, and a very late swing of undecideds.

Pollsters did not immediately have any concrete answers. YouGov’s chief executive apologised on Twitter on Friday morning:

Stephan Shakespeare (@StephanShaxper) A terrible night for us pollsters. I apologise for a poor performance. We need to find out why.

Dr Chris Hanretty, of BBC Newsnight’s Index and the University of East Anglia, said he felt “a little bit foolish”.

“When we said we might be wrong, we talked about being 20 or 25 seats off on the top two parties. We did not think we would be off by more than 40 seats. And we categorically ruled out a majority.

“We’ll be looking at our forecast model and trying to work out what we could have done better. We imagine the polling industry will be doing the same,” he told the BBC. “We should have expected far more ‘shy Tories’.”

Nate Silver, the man once lauded as an elections oracle for his detailed predictions, was wildly out, putting the Conservatives at “about 280 seats, Labour at about 265”.

“The 90% confidence interval on our popular vote forecast had the Conservatives getting as much as 37% of the popular vote and Labour as little as 30% – within the range of the results it now looks like we’ll see,” he blogged on Friday morning. But Labour’s problem was that its share of the national vote increased in areas where it did not win more seats, and shrank in places it needed them, he said.



“Still, forecasters almost certainly ought to have accounted for a greater possibility of an outcome like the one we saw,” Silver said, calling it “a good lesson”.

“Polls, in the UK and in other places around the world, appear to be getting worse as it becomes more challenging to contact a representative sample of voters.”

There were, in truth, a few murmurings that this might happen. Writing on politicalbetting.com on Thursday, GfK NOP associate director Keiran Pedley said he believed there were a number of reasons why polling might be way out.

He pointed to a key group of polled Labour voters who said they preferred Cameron as prime minister – up to 28% in Ipsos Mori’s poll. “Not all party ‘supporters’ in opinion polls are committed activists – some are floating voters liable to change their minds,” he wrote. Those could be the “shy Tories” of 2015.

But is that the real reason behind the massive underestimate of Tory votes, when polls have been adjusted for more than a decade to factor in Tory bashfulness?

“It could be shy Tories, it could be a last-minute swing and a disproportionate number of ‘don’t knows’ going Tory, but it seems odd,” said University of Oxford political sociologist professor Stephen Fisher. “YouGov, for example, make the claim that their polling is ‘faceless’ so there should be no shyness, and the final YouGov poll was done after people had voted, so that doesn’t really account for it.”

Polling blog Number Cruncher Politics raised the possibility that polling methods had not adequately adjusted to the new era of politics. “The unusual fluidity of the electorate in the 2010-2015 parliament may have severely blunted the effectiveness of some of the adjustments introduced after 1992,” the blog noted. “This is a very significant concern for pollsters, some of whom have even gone on record to say so.”

There is a pro-Labour and anti-Conservative bias in polls Stephen Fisher

Using three adjusted statistical models, measuring polling internals, topline numbers and real votes, the site predicted a “much stronger result than current polling averages or forecasts suggest”.

Fisher said polling had consistently overestimated the Labour vote – and that had been happening since the 1970s. “Polling companies have done a lot of work to try to counteract this, but it keeps happening. There is a pro-Labour and anti-Conservative bias in polls.”

That is a point backed by research done by Rob Hayward, a former Tory MP, on the byelections and the local and Euro elections that took place last year and the polling that preceded them.

The Conservative vote was understated by 1.8% and the Labour vote overstated by 3.7% in byelections. In local elections, Labour’s share of the vote was overstated by more than five points, according to the ConservativeHome report on the data.

Ben Lauderdale on the FiveThirtyEight site said he believed constituency polls from Lord Ashcroft that asked about specific MPs rather than parties “performed poorly and overstated the extent to which tactical voting would enable the Liberal Democrats to hold onto seats they would lose on the standard voting-intention question.

“Had we based our forecast on the standard voting-intention question, it likely would have had the Liberal Democrats winning 10 seats,” he said.

Polls are not yet meaningless, Fisher said, pointing to Ashcroft’s polling, which he said had consistently got the narrative correct, even if the numbers were wrong.

“He correctly predicted the pattern of Labour doing well in London, he just got the levels wrong,” he said. “Likewise in Scotland, he got the SNP story correct, but not the numbers. The themes were right, and that’s a cause for optimism for his method of polling.”