Nate Silver has compared pundit analyses of poll data to actual voting results and determined that the pundits get things consistently wrong and in the wrong direction:
This French election was part of a pattern that I began to notice two years ago in elections in the U.S. and elsewhere in the world. Take the 2012 U.S. presidential election as an example. Most of the mainstream media concluded that the race was too close to call, despite a modest but fairly robust Electoral College lead for then-President Barack Obama. But on Election Day, it was Obama who beat his polls and not Mitt Romney.
Forecasters are overconfident more often than they might realize — and there’s a lot to be said for media outlets erring on the side of caution until a vote has taken place. But France was the wrong hill for anything-can-happen-because-Trump! punditry to die upon. Whereas Clinton led Trump by just 3 to 4 percentage points in national polls (and by less than that in the average swing state), and “Remain” led “Leave” by only a point or so, Le Pen had consistently trailed Macron by 20 to 25 points.
Despite their vastly different polling, however, Trump, Brexit and Le Pen had all been given a 10 to 20 percent chance by betting markets — a good proxy for the conventional wisdom — on the eve of their respective elections. Experts and bettors were irrationally confident about a Clinton victory and a “Remain” victory — and irrationally worried about a Macron loss. In each case, the polls erred in the opposite direction of what the markets expected.
Pollsters have a difficult and essential job, but they’re under a lot of pressure from media outlets that don’t understand all that much about polling or statistics and who often judge the polls’ performance incorrectly. They’re also under scrutiny from voters, pundits and political parties looking for reassurance about their preferred candidates. Social media can encourage conformity and groupthink and reinforce everyone’s prior beliefs, leading them to believe there’s a surfeit of evidence for a flimsy conclusion. Under these conditions, it’s easy for polls to be contaminated by the conventional wisdom instead of being a check on elites’ views — and to be the worse for it.
Can't wait to see the polls on the 2018 U.S. elections.