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POLLSTERS often complain that they are not remembered when right, and thoroughly abused when wrong. But British polling firms ought to be exempt from sympathy. Since 1979 they have mismeasured the difference between the Conservatives and Labour, the all-important general-election statistic, by an average of 4 percentage points. In the most recent general election, held in 2015, the final polls missed the result by a shocking 6.6 points—provoking an existential crisis among pollsters and a thorough inquiry by the British Polling Council into their failings. In their next trial, less than a week away, they may once more underperform.

On April 18th, when Theresa May, Britain’s prime minister, announced a snap election to be held on June 8th, pollsters must have considered themselves blessed. Then, according to an average of polls, Conservatives were 19 points ahead of Labour, led by an old-style Leftist, Jeremy Corbyn. But thanks to ill-timed U-turns on Mrs May’s part, that lead has narrowed to a mere six points, and is closing a bit more each day. Pollsters began worrying about their forecasts just two weeks ago, says John Curtice of Strathclyde University. Before that, “all you had to do is put up a big number and everyone thinks you’re right.”

On the morning of June 9th, some pollsters will emerge triumphant—and others bleary-eyed and egg-faced. Over the past week estimates of the Conservatives’ lead have ranged from a sturdy 12 points, from ICM, to an alarming three points from YouGov, which also predicts a hung parliament, in which no party has an overall majority.

Every polling outfit has its own quirks, biases and prejudices, which are together termed “house effects”. For that reason, modellers wishing to forecast elections prefer to work with polling averages. Yet even these have difficulties. In British elections since 1979, Labour voters have been consistently overrepresented by an average of 1.6 points, while Conservative supporters have been underrepresented by 1.1 points. Some think this is because of “shy Tories”—people who are unwilling to admit to pollsters that they plan to vote Conservative, for fear it makes them seem more interested in tax cuts than in protecting the weakest in society. Another theory is that it is the work of “overeager” Labour voters, who tend to be more interested in politics, and thus likelier to respond to surveys.

But where polls have been poor, another indicator has proved itself remarkably helpful to prognosticators. Since the Thatcher era, the party with the more popular leader, as measured by approval ratings, has always won the national vote and formed a government. Gaps in approval are more closely correlated with final results than are polling averages taken even the day before an election—a result first pointed out by George Elliott Morris of the University of Texas using data from Ipsos MORI, a pollster. Unhelpfully for Mrs May, her large popularity lead over Mr Corbyn has plummeted, along with the number saying they will vote Conservative.

At the start of the campaign, 46% of Britons had a favourable view of Mrs May, compared with 21% for Mr Corbyn. He was the least well-regarded leader of either major party in nearly four decades, with the exception of Michael Foot, another far-leftist, who led Labour to catastrophic defeat in 1983. Judging by the run-up to previous elections, winning the popularity contest by such a wide margin would portend an eventual victory of some 13 points.

But the latest figures from Ipsos show that satisfaction with Mrs May has dropped 14 points, as it has risen 8 points for Mr Corbyn. She now has just a four-point advantage—which still would suggest a slim 3.5-point Conservative victory, though with a wide margin of error. Mrs May must wonder whether it was worth calling an election at all.