All polling of elections is subject to many potential sources of error. The historical record at general elections indicates that there is a one in ten chance that an individual poll will over or underestimate a party’s support by more than four points.
These European Elections presented particular challenges. Not only did pollsters have to face the challenge of an election in which turnout was likely to be low (and in the event stood at 36.7%), but also of estimating the level of support for two new parties, one of which went on to come top of the poll.
The table below compares the estimates of BPC members’ polls whose fieldwork ended no earlier than three days before polling with that of the actual result — though in two instances fieldwork commenced more than a week before polling day.
It reveals that on average the polls overestimated support for Labour and underestimated that for the Liberal Democrats and the Greens. However, two polls produced estimates for Labour and the Liberal Democrats that were within a point of the eventual outcome, while one was spot on in its estimate for the Greens.
There was also a less marked tendency to overestimate support for the Conservatives and the Brexit Party, and in both these instances one poll was spot on in its (integer) estimate of party support.
The detailed council by council election results showed that turnout increased more in Remain-voting areas than in Leave-voting ones, suggesting that those who voted Remain were more likely to have voted than did those who voted Leave. Meanwhile, polling conducted by Opinium on polling day suggests that those who switched from voting Labour in 2017 to either the Liberal Democrats or the Greens were more likely than most voters to have made up their minds about how to vote in the last week of the campaign. These patterns may help to explain at least some of the average error in the polls on this occasion.
| Con | Lab | LD | Brexit | Green | UKIP | Change UK | Other | METHOD | SAMPLE SIZE | FIELDWORK | ||
|---|---|---|---|---|---|---|---|---|---|---|---|---|
| Opinium | 12 | 17 | 15 | 38 | 7 | 2 | 3 | 4 | Online | 2005 | May 17-20 | |
| YouGov | 7 | 13 | 19 | 37 | 12 | 3 | 4 | 6 | Online | 3864 | May 19-21 | |
| Panelbase | 12 | 25 | 15 | 30 | 7 | 3 | 3 | 5 | Online | 2033 | May 14-21 | |
| Kantar | 13 | 24 | 15 | 27 | 8 | 4 | 5 | 4 | Online | 2316 | May 14-21 | |
| Ipsos MORI | 9 | 15 | 20 | 35 | 10 | 3 | 3 | 6 | Telephone | 1527 | May 20-22 | |
| BMG Research | 12 | 18 | 17 | 35 | 8 | 2 | 4 | 5 | Online | 1601 | May 20-22 | |
| Survation | 14 | 24 | 12 | 32 | 7 | 4 | 3 | 6 | Online | 2029 | May 22 | |
| Average | 11.3 | 19.4 | 16.1 | 33.4 | 8.4 | 3.0 | 3.6 | 5.1 | ||||
| Result | 9.1 | 14.1 | 20.3 | 31.6 | 12.1 | 3.3 | 3.4 | 6.1 | ||||
| Difference | 2.2 | 5.3 | -4.2 | 1.8 | -3.7 | -0.3 | 0.2 | -1.0 | ||||
Note: Table is based on the outcome in Great Britain. The polls conducted by Opinium and Survation were conducted across the UK as a whole, but the figures for those polls quoted here are for respondents in Great Britain only.