Hillary Clinton is heading for a landslide victory over Donald Trump. But wait. Trump is pulling ahead and could take the White House. No, Clinton has a clear lead and is gaining ground. Nearly every day, a new poll comes out touting a different result, leaving voters wondering what to believe.

The results of recent elections give even more reason for scepticism. In 2013, the Liberal Party of Canada confounded expectations when it won the provincial elections in British Columbia. The following year, polls overestimated support for Democrats in the US congressional elections. And this year, some pollsters underestimated Britons’ support for leaving the European Union in the Brexit referendum. These blunders have led some political commentators to say that polls are headed for the graveyard.

“It’s harder and harder to find people willing to pay for any polls, given their poor performance this year and last year. They’re heavily discredited in the UK,” says Stephen Fisher, a political sociologist at the University of Oxford.

As the US presidential election approaches, pollsters are scrambling to improve their methods and avoid another embarrassing mistake. Their job is getting harder. Until as recently as ten years ago, polling organizations were able to tap into public opinion simply by calling people at home. But large segments of the population in developed countries have given up their landlines for mobile phones. That is making them more difficult for pollsters to reach because people will often not answer calls from unfamiliar numbers.

So the pollsters are fighting back. They are fine-tuning their efforts in reaching mobile phones, using statistical tools to correct for biases and turning to online surveys. The increasing number of online polls has prompted the formation of polling aggregates, such as FiveThirtyEight, RealClearPolitics and Huffington Post, which combine and average the results to develop more nuanced forecasts.

The mobile revolution has hit pollsters hard in the United States because federal regulations require that mobile phones be called manually. And people often do not answer calls to their mobiles when an unfamiliar number pops up. In 1997, pollsters could get a response rate of 36% but that has dropped to just 10% or less now. As a result, pollsters are struggling to reach as many people, and costs are going up: each mobile-phone interview costs about twice as much as a landline one. There is also a ‘non-response bias’, because people who respond to pollsters’ calls sometimes do not reflect a representative sample, says Frederick Conrad, head of the Program in Survey Methodology at the University of Michigan in Ann Arbor.

Read the full article "The polling crisis: How to tell what people really think" at Nature.