Four ways polls could be wrong

If the polls are correct, this election is likely over.

In HuffPollster's estimate, Hillary Clinton is winning by 8 percentage points right now, based on twocandidate surveys. That edge looks close to insurmountable with three weeks remaining and with voters in many states already casting their ballots.

This is when partisans start speculating that the polls are getting it wrong. Here are some variables. Third-party craziness. Polls show third parties doing unusually well in this cycle, with Gary Johnson at 7 per cent nationally. In normal years, third-party candidates underperform their polling numbers on Election Day. This leaves pollsters (and forecasts based on surveys) with a dilemma. If they include Johnson (and the Green Party's Jill Stein and independent conservative candidate Evan McMullin) in their survey, they probably overestimate votes for third-party candidates. Shy voters. Trump's supporters argue he will benefit from “shy” voters who are hesitant to admit they will cast their ballots for him. That's plausible. But so is the possibility that some people (perhaps in red states) might be reluctant to admit to supporting Clinton. Organisation. By all accounts, the Clinton campaign and the Democratic Party have a large advantage in field operations over the Trump campaign and the Republican Party. This should mean that, all else being equal, a Clinton supporter will be a bit more likely to cast a ballot than a Trump supporter. Lots of voters intend to vote but never actually do; this is one reason polling is difficult to begin with. Maybe this unknowable helps Trump. Or maybe it gives Clinton an advantage not measured in polls, especially in swing states, where campaign organisation is most intense. People are casting ballots each day. Therefore, the trick for pollsters is less and less about predicting what people will do on election day, and more and more about figuring out what people have already done. Technical stuff. Many people find it hard to believe that the opinions of a few thousand Americans interviewed in polls can predict the actions of millions of voters. Yet the maths of these surveys works. We've seen plenty of examples of polls successfully predicting election results. Even this year in the primaries the polling was fairly good, though they are harder to measure than general elections.

But even experienced professionals can make mistakes, and not everyone publishing survey results is a top-rated pro.

Polling averages (the basis for all the poll-based forecaster estimates) are good in part because the decisions pollsters make tend to cancel each other out. But if there are unpredictable changes in the electorate then pollsters may miss them.

While all of these effects are plausible, the most likely result is that the polls will wind up being correct.

— Bloomberg