UK General Election polls: Why the polls are varying so much… and why under-25s could decide this election

Facing off: Theresa May and Jeremy Corbyn
Peter Kellner30 May 2017
WEST END FINAL

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One message cries out from the latest polls – and it’s NOT that they are all over the place and should be ignored. It is that turnout holds the key to the election result.

Consider the evidence. At one extreme, ORB and Survation say the Conservatives are just six points ahead, and Theresa May might struggle to maintain the tiny majority she inherited from David Cameron. At the other extreme, ICM puts the Tories 14 points ahead and on course for a landslide.

In theory those differences could simply reflect random sampling fluctuations. But that is not the case here. In fact, all six polls paint a remarkably similar picture of Britain’s electorate. After weighting their raw data to match the political and demographic make-up of the country, they all show the Tories just 5, 6 or 7 points ahead.

Their published figures vary because of what they do next. Pollsters know that some people are more likely than others to take the trouble to vote. The six polls differ because they deal with this turnout factor in different ways.

ORB, YouGov and Survation all reckon that this factor makes little difference to the Conservative lead.

In contrast, the turnout factor adds three points to Opinium’s Tory lead, while ComRes and ICM make politically huge adjustments – seven and nine points respectively. For example, their methods produce a far lower turnout among younger voters than other polls. As this group is strongly Labour, this tweak has the effect of reducing Labour’s overall vote share and widening the Tory lead.

This is all very different from two years ago, when all the polls achieved politically similar samples, made similar turnout adjustments – and all got the result wrong.

At the end of next week, we shall discover which polls (if any) are right, and which are wrong. The big message from the latest polls, though, not just a technical one. It is that turnout matters hugely this time. The relative success or failure of each party to get its supporters, especially the under-25s, to the polling stations next week could decide whether Theresa May obtains the kind of mandate she seeks.

Meanwhile, I have edged down my projection of the Conservative lead, matching the broad trends of most recent polls. But in this, the most volatile election of recent times, there could well be more shifts to come, in either direction.

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