General election 2017: Corbyn confirms Labour would abolish zero-hour contracts – politics live

21 Apr

On average, UK polls this far out have missed the final margin by 6 percentage points. And they don’t get all that much more accurate as you go along — the final polling average has missed the result by 5 points. The experience in Brexit last year — when the polls missed the final margin by 4 points according to the Huffington Post polling average or 6 points according to the method I described above — wasn’t a big outlier by UK standards. The same goes for the previous U.K. general election in 2015, when they underestimated Conservatives by around 6 points. Polls in 2010 were quite good in diagnosing the Conservative-Labour margin, although they considerably overestimated Liberal Democrats’ performance.

May’s Conservatives do have a massive lead, with recent polls showing them 9 to 21 points ahead of Labour and their unpopular leader, Jeremy Corbyn. Also, while the polls in the UK haven’t been very accurate, they’ve tended to underestimate Conservatives rather than Labour in the past.

But if polls are missing election outcomes by 5 or 6 points on average, that means the margin of error (or 95 percent confidence interval) is very large indeed. Specifically, a 6-point average error in forecasting the final margin translates to a true margin of error of plus or minus 13 to 15 percentage points, depending on how you calculate it.

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