European election polling post-mortem
To start with, here's an update of all the pre-election polls (Ipsos MORI, Survation and NCPolitics all published theirs on the morning of election day, after my last post).
Note that ComRes and Hanbury also produced polls during the campaign, but not with fieldwork conducted on or after the final weekend of the campaign. For what it's worth, they tended to show high Labour support, though we'll never know what their polls would have shown in the final week.
Needless to say, the pre-election polls varied wildly from one another for all the main parties. Labour had a twelve point spread (13% to 25%), the Conservatives eight points (7% to 15%), the Liberal Democrats (12% to 20%), the Brexit party eleven points (27% to 38%). In the event, the polls that had low Labour scores and high Liberal Democrat scores were closest to reality. Compared to the final results, Ipsos MORI took the laurels, getting close to the correct result for all parties. YouGov were next, getting within a point or two of most parties but overstating the Brextit party. Other companies recorded significant errors, with a few double-digit overstatements of Labour support.
It is difficult to point at a single obvious cause for the wide variation. When there were huge differences between polls at the 2017 election the reasons the were clear: pollsters had adopted demographic turnout models and other post-fieldwork adjustments which backfired and overstated Tory support. There is no such easy explanation for the 2019 polls - pollsters have mainly reversed the missteps of 2017 and, while there are some variations in approaches to turnout, the elaborate turnout models that made such a difference in 2017 have disappeared. Different approaches to turnout perhaps explain differences of a point or two, they don't explain differences of 10 points. The differences here look as if they are more likely to be down to pollsters' different approaches to sampling or weighting, and the representativeness of their samples.
From the beginning these European elections were going to be a challenge. They are a low turnout election, when at recent elections polls have struggled to correctly reflect the demographic pattern of turnout. In recent decades most British pollsters have also relied upon past-vote weighting to ensure their polls are politically representative, and this was an election when past vote was a particularly poor predictor of current voting intention.
In terms of what this means for wider polling, errors here don't necessarily transfer directly across to Westminster polls. The challenges posed by high-turnout elections can be very different to those posed by low-turnout elections and just because some polls overstated Labour support in the European elections does not necessarily mean they are overstating Labour support for general elections. On the other hand, given the recent history of errors, it probably isn't something we in the polling industry should be complacent about.
