FEATURE POLLS APART
Polls apart T
Why didn’t the opinion polls before the 2015 General Election predict the actual election result? Jon Mellon and Chris Prosser of the British Election Study team examine some of the possible causes of the polling miss and what polling companies can do to prevent it in future
HE POST-ELECTION wave of the British Election Study (BES) Internet panel allows us to take a closer look at possible causes of the polling miss during the
recent General Election. In this article we outline the evidence we have gathered so far to test five possible explanations: 1) ‘don’t knows’ shifting, 2) a late swing among voters, 3) Shy Tories, 4) problems achieving a representative sample and 5) differential turnout. Late swing and ‘don’t knows’ The post-election data immediately casts doubt on two of the theories. In our campaign wave, seven per cent of people said that they ‘don’t know’ who they would vote for. In the post-election survey (when we can see how undecided respondents ended up voting), we find a very small edge for the Conservatives among previously undecided voters. However, ‘don’t knows’ only contribute around 0.05 percentage points towards the polling gap so it is unlikely to have been a major factor. Similarly, there is no difference between the proportion of respondents supporting the Conservatives in the campaign wave and the post-election wave, making it unlikely that there was a late swing. Shy Tories We also have evidence against the Shy Tories theory. We can examine the Shy Tories theory by considering where there is likely to be social pressure on Conservative voters. For example, it seems unlikely that Tories would need to be shy
“ It seems unlikely that Tories
would need to be shy in the heavily Conservative Shires
in the heavily Conservative Shires but it is more plausible that they would be shy in traditional Labour heartlands like Sunderland. We actually observe the opposite pattern. The deviation between the proportion of BES respondents saying they voted Conservative and the actually proportion of voters who did is highest in strong Conservative areas where we would expect the least social pressure against voting Conservative. There is also no apparent evidence for another
aspect of the Shy Tories theory. Several pollsters have suggested that placing the vote intention question later in a survey makes respondents more willing to admit that they plan to vote Conservative. In the first three waves of the BES we randomised the placement of the vote intention question to be
22 SOCIETY NOW SUMMER 2015 ”
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40
Age BES-Census difference in age %
60
80 Con-Lab lead
Deviation in proportion of respondents of each age compared to census (left axis). Vertical lines designate the age boundaries of the weighting age groups. The red line is a LOWESS regression of Conservative-Labour lead against age among BES respondents in wave 6 (right axis).
either at the start of the survey or at the end after all other questions and the question placement makes no difference to the proportion of respondents intending to vote Conservative. Taking these findings together, we are doubtful whether Shy Tories were a major contributor to the polling miss. Representative samples We also have more evidence about the representativeness of polling samples used before the election. One possible source of non- representativeness could be the groupings used for weighting by polling firms. For instance, do the age groupings used for weighting (both by YouGov and many other polling firms) hide variation within those groups? The red line in the graph above shows the lead in share of the vote that the Conservatives have over Labour by age. The vertical lines represent the breakpoints between the standard age bands used for weighting. The bars show the difference between the percentage of BES respondents of a particular age and the percentage of the 2011 population of the same age (eg, positive bars mean that an age is over-represented in the BES). The most important deviation is the oldest age group, where younger (less Conservative-leaning) respondents are overrepresented whilst older (more Conservative leaning) respondents are underrepresented. The net effect of this difference is to dampen the Conservative lead. This problem
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BES-Census difference in age % 5
-5 0
1
-20
-10
Con-Lab lead 0
10
20
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