FiveThirtyEight: Are The U.K. Polls Skewed?

Conservatives won only a slim overall majority in 2015 despite winning the popular vote by 6.5 percentage points because 87 seats went to regional parties (especially the Scottish National Party, which won 56 seats), third parties (such as Liberal Democrats) or independent candidates. While FiveThirtyEight isn’t attempting to translate votes to seats — that’s a tricky problem and one that we haven’t had much luck with in the past — other people’s models show that if Conservatives were to win by much less than their 2015 margin, their majority would be under threat. A series of YouGov models released this week have shown Conservatives winning 308 to 317 seats — short of a 326-seat majority — with a 3- to 4-point win. Uniform swing calculations also suggest that Conservatives would be underdogs to retain their majority with a 4-point win, and about even money to do so with a 5-point win. [...]

Exactly how strong the Conservative tendency to outperform their polls has been depends on where you measure from. Since 1992, Conservatives have beaten their final polling margin over Labour by an average of 4.5 percentage points, and have done so in all but one election. (That was 2010, when both Conservatives and Labour gained ground as Liberal Democrats’ support collapsed, but Labour slightly outperformed its polling margin against the Tories.) Go all the way back to 1945, however, and the average Conservative overperformance is just 1.8 percentage points and is not statistically significant.4 [...]

This is also the case in Europeanwide polling. It’s often assumed that nationalist or right-wing parties tend to beat their polls, perhaps because (as is supposedly the case with Shy Tory voters) people are reluctant to declare their support for a “politically incorrect” party to pollsters. But over dozens of European elections over the past several years, there’s been no systematic tendency for nationalist parties to outperform their polls. Yes, it happens sometimes — such as in the 2015 election in Denmark. But the nationalist party has underperformed their polls almost exactly as often, as Marine Le Pen and the National Front did last month in France.

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