On 28th September, Rt Hon Nicky Morgan MP, Chair of the Treasury Committee, wrote to Professor Sir John Curtice, President of the British Polling Council. The letter focuses on the use of private polling data, particularly during the recent referendum campaigns, and follows on from media coverage during the summer. Further details, including a link to Mrs Morgan’s letter, can be found on the Treasury Committee’s web page: Change polling rules to protect integrity of UK financial markets.

Following a consultation with its membership, the British Polling Council has responded to Mrs Morgan. The contents are set out in this letter to the Rt Hon Nicky Morgan MP (pdf).

Seven member companies issued ‘final’ polls of voting intentions in the EU referendum. While no company forecast the eventual result exactly, in three cases the result was within the poll’s margin of error of plus or minus three points. In one case Leave were correctly estimated to be ahead. In the four remaining cases, however, 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.

Polling of voting intentions in the referendum was a unique event, and the lessons of this experience will not necessarily be transferable to polling of future general elections. Consequently, the BPC does not believe that a full-scale independent inquiry, similar to that set up after the 2015 General Election, would justify the large amount of time involved. However, the Council will be 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 commented, ‘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.’

The following table shows the results of all polls conducted by BPC members that had at least some fieldwork within the last 4 days of the campaign.

As well as the pre-election polls there were three published on-the-day polls, by BMG, YouGov and Ipsos MORI. As they combine people who are reporting their actual vote and others who are reporting their voting intention they are treated separately from the pre-election polls above. 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. BMG interviewed 2,000 people online with a declared result of Remain 46% Leave 41% and Don’t know 13%.

The ORB and Ipsos MORI polls covered only Great Britain. All the other polls above covered the whole of the UK.

British Polling Council member TNS-BMRB also conducted a poll within the fieldwork period above. However, they took the decision not to remove and reallocate in any way those who said they were undecided, or that they would not vote, and so no calculation of error is possible. For the record, their final poll was conducted online from 16-22 June, with a sample size of 2,320, and the results were Remain 41% Leave 43% Undecided/would not vote 16%

In the interests of completeness, British Polling Council member BMG Research conducted two polls, one telephone and one online, that finished over a week before polling day, and which are therefore not included in the table above. Fieldwork was conducted from 10-15 June, with sample sizes of 1,043 (telephone) and 1,468 (online). The results were Remain 53% Leave 47% (telephone) and Remain 44% Leave 56% (online).

British Polling Council member LucidTalk conducted a poll solely in Northern Ireland. Fieldwork was conducted online from 15-17 June, with a prediction of Remain 57% Leave 43%. As the actual Northern Ireland result was Remain 56% Leave 44% this represents an error of only 1% in its estimate of the vote.