BPC verdict on EU referendum polls

“In the four remaining cases, support for Remain was clearly overestimated. This is obviously a disappointing result for the pollsters, and for the BPC, especially because every single poll, even those within sampling error, overstated the Remain vote share,” said the BPC statement.

Because the referendum is a unique event – the lessons of this experience will not necessarily be transferable to polling of future general elections – the BPC will not be embarking on a full-scale independent inquiry, as it did after the 2015 General Election.  

However, it is asking all its members to look carefully at the methods that they used in the referendum and to report their findings, and will consider inviting an external reviewer to consider the methods that were used.

BPC president John Curtice said: “Polling of voting intentions in this referendum was always going to be a difficult exercise. Yet their central message – that this looked like a close referendum that neither side could be sure of winning – proved prescient. Nevertheless, there will be an obligation on members to try and establish why there was a tendency to overestimate support for Remain, and the Council will report later this year on why that appears to have been the case.”

As well as the pre-election polls there were two published on-the-day polls, one by YouGov and one by Ipsos Mori. They are treated separately from the pre-election polls because they combine people who are reporting their actual vote and others who are reporting their voting intention.

The YouGov poll interviewed 4,772 people online and produced a result of Remain 52% Leave 48%. Ipsos MORI interviewed 546 people by telephone and produced a result of Remain 54% Leave 46%. This is a smaller sample size than their standard published polls as it was not intended as a standalone on-the-day poll, and only presented as a day-by-day comparison drawing on their final poll. 

In its own statement, YouGov said: "Voting intentions have fluctuated in this extremely close, unusually volatile, contest. In the end, Leave won the day, something that YouGov showed was a real possibility for the entire referendum campaign. YouGov, like most other online pollsters, has said consistently it was a closer race than many others believed and so it has proved. While the betting markets assumed that Remain would prevail, throughout the campaign our research showed significantly larger levels of Euroscepticism than many other polling organisations. Indeed, the majority of our polls since the start of June showed Leave ahead.

"We do not hide from the fact that YouGov’s final poll miscalculated the result by four points. This seems in a large part due to turnout – something that we have said all along would be crucial to the outcome of such a finely balanced race. Our turnout model was based, in part, on whether respondents had voted at the last general election and a turnout level above that of general elections upset the model, particularly in the North."