In past referenda polls tend to overstate the change option

In past referenda polls tend to overstate the change option

Stephen Fisher and Alan Renwick have analysed past referenda in the UK, and they notice that

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.

In some cases there are substantial trends in public opinion over the last four weeks before voting day. When there is a trend it is typically downwards, i.e. towards the status quo (though not in all cases). So polls one month out have more often shown greater support for change than the polls closest to referendum day.

However, even polls late in the campaign tend to overestimate the eventual support for change. In seven out of the ten cases, the average of the polls in the final week saw greater support for change than the final result. Across all ten cases the average error of the average of polls in the final week overestimated the change vote by 2.25 percentage points. This may be because Don’t Knows disproportionately spilt in favour of the status quo, or it may reflect some late swing or some other reason.

Readers familiar with our earlier writings (e.g. here, here and here) will be aware of these tendencies. But the problem with tendencies sometimes when considering accuracy of opinion polls is that one or more polls may have done reasonably well even if the average poll did not. This is clearly the case in some of these referendums.

What is remarkable though is the extent to which the poll that came closest was at one of the extremes of the range of final-month polls. This was the case in seven of the ten cases….

But before Leavers get despondent, Fisher & Renwick note

….But there is, of course, also reason for caution. The referendum on tax-raising powers for the Scottish parliament shows it is also possible that Leave could outperform every poll. And, indeed, this referendum could generate a pattern not previously seen in a UK referendum. Since we have already seen polls pointing towards opposite outcomes within a month of the vote, the past experience of UK referendums provides no basis for ruling out a Leave victory.

Meanwhile this poll should alarm Cameron and Remain, almost half of Labour supporters don’t know Labour is backing Remain. Which is problematic as Remain and Cameron needs Labour voters to come out and back Remain

The latest  YouGov poll fails to show any movement to Leave that the ICM and ORB phone polls did


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