FEATURE POLLS APART
and trade union membership, as well as several constituency factors, including the overall turnout in their constituency in the previous General Election. After accounting for these factors, we estimate that our respondents’ turnout is likely to have actually been around 73.4 per cent. Importantly, we can look at how vote intention differs among respondents who have different predicted probabilities of voting. The Labour lead among unlikely voters grew
hugely between 2010 and 2015, suggesting that differential turnout is an important factor in explaining the polling miss: considerably fewer of those saying they were going to vote Labour are likely to have actually turned out to vote. Reweighting our respondents according to their predicted probabilities of voting explains about
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is even greater for the oldest respondents in the sample – those over age 80 make up 5.1 per cent of the population, but only 0.5 per cent of the BES. This evidence suggests there is some pro- Labour bias due to the age groupings used, but this might yet be cancelled out by other parts of the weighting scheme. We will need to examine all the weighting variables before we can draw conclusions about the contribution of non- representative samples to the polling miss. Differential turnout There is also new evidence for the differential turnout theory. Nintey-one point six per cent of our respondents claim to have voted compared with 66.4 per cent in Great Britain as a whole. While this partially reflects the fact that polling respondents tend to be more politically interested than the general population, we also have considerable evidence that respondents overstate their turnout: 20 per cent of respondents in areas without local elections claim to have voted in them in 2015; 3-6 per cent of respondents in the campaign wave claim to have voted by post before the postal ballots were actually issued and 46 per cent of respondents who we have confirmed were not registered to vote in June 2014 claim to have voted in the 2014 European Elections. In all of these cases, the fibbers lean significantly more toward Labour than other respondents. We look at the impact of overstated turnout more precisely by building a predictive model of turnout based on the validated vote in the 2010 BES face-to-face survey. The model accounts for a respondent’s stated likelihood of voting prior to the election, turnout in previous elections, their age, marital status, household income, unemployment
Why did
the General Election result contradict the majority of pre-election opinion polls?
The evidence suggests there
is some pro-Labour bias due to the age groupings used
25 per cent of the gap in the Conservative lead between the pre-campaign wave of our survey and the actual election results. The evidence in the BES suggests that the reason for the increased impact of differential turnout is not due to a change in the relative enthusiasm between Labour and Conservative supporters since 2010. Eighty-four per cent of Labour supporters in 2015 said that it was ‘very likely’ that they would vote, compared to 86 per cent of Conservative supporters, while in 2010 the figures were 87 per cent and 90 per cent respectively. Rather the data suggest that the increase in the turnout gap between Labour and the Conservatives can be explained by shifts in party support amongst those who are actually less likely to turn out to vote, even if they say they will. This evidence strongly suggests that differential turnout was a major factor in the polling miss. If differential turnout is the primary cause of
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the polling problems, this is relatively good news for pollsters. It should be possible for pollsters to fix many of their surveys by using turnout weighting that accounts for the wider set of factors we have identified. Our analysis of the post-election BES data makes us much more sceptical about late swing, ‘don’t knows’ and Shy Tories. By contrast, we are leaning very strongly towards differential turnout as an explanation and think that it’s likely that sampling and weighting played at least some role. n
i
The British Election Study 2015 is managed by a consortium of The University of Manchester, The University of Oxford and The University of Nottingham. The Scientific Leadership Team comprises Professors Ed Fieldhouse, Jane Green, Hermann Schmitt, Geoff Evans and Cees van der Eijk. The team is supported by researchers Dr Jon Mellon and Chris Prosser and also by BES 2015 consultant Professor John Curtice (University of Strathclyde).
www.britishelectionstudy.com SUMMER 2015 SOCIETY NOW 23
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