Polling failures 'changed outcome of 2015 general election'

Polling failures changed the outcome of the general election, senior politicians have claimed after an inquiry found surveys had relied on unrepresentative samples.

Experts found that Tory voters were under-represented in telephone and online studies and said they could not rule out the possibility of "herding" - pollsters designing their surveys and weighting responses in such a way that their results were closely in line with those of rival organisations.

Former Liberal Democrat leader Lord Ashdown, who dismissed the election night exit poll with a promise to eat his hat if it was true, said the polls had a "considerable" impact on the way people voted.

He told BBC Radio 4's World At One programme: "The effect of the poll was to hugely increase the power of the Conservative message and hugely decrease the power of the Liberal Democrat message, which was you need us to make sure they don't do bad things."

The peer claimed the "mood of the nation" was for another coalition and voters were "surprised" when the Conservatives won outright.

"I think, therefore, there is an argument to be made that it actually materially altered the outcome of the election."

Lord Ashdown said he was "instinctively" against banning polling in the run-up to the election but warned that pollsters must "get their house in order now".

Some Labour supporters claim that inaccurate polling could have swayed the result of the election, by ensuring that attention was focused on the possibility of a Labour coalition with the Scottish National Party, rather than the agenda of a Conservative-only government.

Former culture secretary Ben Bradshaw said polling had distorted the results of the election.

He tweeted: "Commentariat too relaxed re pollsters' failure. Affected result. Election dominated by hung parliament talk instead of likely Tory majority."

The independent inquiry, chaired by Southampton University professor of research methodology Patrick Sturgis, was commissioned by the British Polling Council (BPC) and the Market Research Society (MRS) after polls suggesting a neck-and-neck race were confounded by the Conservatives' comfortable 6.5% victory margin on May 7.

Prof Sturgis's findings chime with a separate study released last week by polling expert Professor John Curtice, which concluded that sampling methods may have resulted in too many Labour supporters being questioned.

The inquiry's preliminary report, being launched at the Royal Statistical Society in London, has found that unrepresentative samples were the "primary" cause of the polls' inaccuracy.

Methods of sample recruitment used by the polling organisations resulted in "systematic over-representation of Labour voters and under-representation of Conservative voters", which was not adequately mitigated through weighting of responses.

Other potential issues, such as misreporting of voter turnout, question wording or the treatment of overseas voters, postal voters and unregistered voters were likely to have made "at most a modest contribution" to skewing results.

The inquiry will make recommendations to the BPC and MRS when it publishes its final report in March.