How did the opinion polls in the US get it wrong – again?

Joe Biden has been consistently well ahead of Donald Trump for months, according to pollsters, but in the end it is coming down to the wire. We’ve been here before, writes John Rentoul

Joe Biden and Donald Trump are in a tight battle to win the 2020 US presidential election (AFP via Getty Images)

We all have our own shortcuts for trying to understand uncertainty. I find it hard to grasp election predictions expressed as percentage probabilities. When Nate Silver’s 538 website issued its final forecast – an 89 per cent chance that Joe Biden would win – I did not know how to handle that information. It now looks as if Biden will win, but this forecast didn’t seem to capture how close the election would be.  

I prefer to look at state-by-state opinion polls, because it is a state-by-state election. As we learned painfully four years ago, the national vote does not decide the presidency – the votes in a handful of swing states do.  

So I was aware that the election was close. Biden’s lead in the opinion polls in the states he needed to win was on average less than five percentage points. In 2016, Hillary Clinton had a comparable lead of 3.6 points in opinion polls in the key states, and it turned out that those polls overstated her support. If Biden was in a better position, he was only marginally so.