“The fibbers lean significantly more Labour than other respondents”

A new, but still preliminary, tranche of analysis is out from the British Election Study on why the 2015 general election polls were so wrong:

We previously identified 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…

The post-election data immediately casts doubt on two of the theories. In our campaign wave, 7% 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, we find that 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…

We also have evidence against the Shy Tories theory … The deviation between the proportion of BES respondents saying they voted Conservative and the actual proportion of voters who did is highest in strong Conservative areas where we would expect the least social pressure against voting Conservative…

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…

The most important deviation [between the BES sample and reality] is the oldest age group, where younger (less Conservative leaning) [members of the oldest age band] are overrepresented whilst older (more Conservative leaning) respondents are underrepresented. The net effect of this difference is to dampen the Conservative lead…

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. Re-weighting our respondents according to their predicted probabilities of voting explains about 25% of the gap in the Conservative lead between the pre-campaign wave of our survey and the actual election results…

The fibbers lean significantly more Labour than other respondents.