Referendum polling in the UK has historically overstated support for change

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The differing outcomes produced by online and telephone polling during the EU referendum have led to a debate about the merits of each method. But perhaps neither is a good indicator of the final outcome. Stephen Fisher and Alan Renwick suggest that there is often a trend towards the status quo during referendum campaigns in the UK, and that even polls in the final week before a referendum have often shown more support for change than the result.

As online and telephone polls for the EU referendum continue to tell different stories about the contest, there is increasing debate about the relative merits of each method (e.g. here, here and here). Much of this debate is focused on which of the two modes is more accurate. Does Remain have a comfortable lead, as phone polls suggest, or is it too close to call, as the online polls indicate?

Perhaps neither is a good guide to the final outcome. In this post we reflect on the historical experience of polls for referendums in the UK. The graph shows the levels of support for the change option (excluding don’t knows) in polls and the final outcome for all ten referendums in the UK for which there was more than one poll in the final 30 days of the campaign.

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EU referendum forecast update: 79 per cent chance of Remain winning

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Stephen Fisher and Alan Renwick have developed a method for forecasting the outcome of the EU referendum based on current vote intention polling and analysis of opinion polling from previous referendums in the UK and across Europe. Since the last update two weeks ago the probability of a Remain win has increased from 72 per cent to 79 per cent.

There has been a small shift towards Remain in the polls over the last two weeks. Excluding don’t knows, our polling average for Remain has moved from 52 per cent on 10th May to 53 per cent now. This figure is based on the most recent polls from each of seven companies: one from each but two from ICM (one by phone and one conducted online). The Remain share has been adjusted down by 2.15 points for telephone polls and up by the same amount for online polls to account for the relatively stable gap between these different methods in the levels of support they tend to give the two sides.

Using the historical experience of referendum polls and referendum outcomes in the UK and on the EU elsewhere, as discussed here, our latest forecast is for Remain to win 55 per cent of the vote in a month’s time. The 95 per cent prediction interval surrounding this estimate has narrowed very slightly to ±12.5 points. So we are forecasting that Remain will win between 43 per cent and 68 per cent of the vote.

Values closer to the middle of this range are more likely. Overall the probability that the Remain vote will be larger than the Leave vote is now 79 per cent, up from 72 per cent two weeks ago.

This post was originally published on Elections Etc. and is re-posted with permission.

About the authors

Dr Stephen Fisher is an Associate Professor in Political Sociology and the Fellow and Tutor in Politics at Trinity College, Oxford.

Dr Alan Renwick is the Deputy Director of the Constitution Unit.

How to win EU referendums: lessons from the continent

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Ece Özlem Atikcan, Assistant Professor in Political Science at Laval University, asks whether previous referendums on European Union treaties contain many lessons and insights into the upcoming referendum in the UK. This piece is part of the UCL European Institute’s first Brexit Divisions guest editor week on openDemocracy, from which a selection of the posts will also be published here.

Referendum campaigns matter more than election campaigns. Research has shown time and time again that people tend to change their minds during a referendum campaign, especially when the subject is an unfamiliar one, and the politicians line up in a non-traditional way. Voters then rely on campaign information to make sense of the referendum proposal, and the way political actors present the issue makes a difference.This is typically the case in referendums on the European Union (EU), and even more so when the referendum question concerns EU treaties, which are long and technical. Besides being unfamiliar with the EU’s unique terminology, European citizens lack direct interaction with EU institutions in their daily lives.

In addition, these campaigns often pitch the parties in the middle against those that are at the extremes of the political spectrum. The far left and the far right come together in their fight against the proposal, forming alliances never seen in regular elections. Referendum campaigns thus have an important role in helping citizens make sense of the European treaty at hand. In the referendums on Maastricht, Nice, Constitutional and Lisbon Treaties, campaign information has been shown to be the key to understanding the vote choice.

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