Was Donald Trump’s win unexpected? Not if you followed the betting markets, which had Trump at a two-thirds chance of winning days out from the election. The polls, on the other hand, told a different story. Analysis of polls carried out in 15 competitive states in the three weeks before last Tuesday’s election shows that whatever the method of polling used, there was a clear and consistent bias in favour of the Democrats.

Pollsters spent an estimated half a billion dollars (£388 million) on this election, but most polling methods were still biased towards Kamala Harris by around three percentage points....