There is a problem with tomorrow’s exit poll. Ipsos Mori will be collecting data from voters as they leave 144 of the 50,000 polling stations around the country. This may produce a relatively accurate tally of those voting on the day.

But more than a fifth of voters will be excluded, because they have already voted by post. If there is a variance between postal voters and ‘in person’ voters, the exit poll will not pick it up.

Are postal voters any different from those who turn up at the polling station? On past evidence, they are more likely to vote. In 2017, 85 per cent of postal voters cast their vote, compared to 66 per cent of those voting in polling stations. Given the forecast for bad weather tomorrow, that variance may prove starker this time.

Given the forecast for bad weather, the variance in postal voting may prove starker than usual

By contrast, regional variation is not huge – in 2017 most regions had a variance within 3 percentage points of one another, with a national average of 18 per cent. The exceptions are Northern Ireland (1.9 per cent) and the North East (26.6 per cent).  With three Labour-held super-marginals in this region, a variance between method of voting in Stockton South, Bishop Auckland, and Darlington will only tip the election result if the nationwide picture is truly down to the wire. 

For the nationwide exit poll, one option would be to factor-in a proportionate figure to account for where the opinion polls stood over a week ago when many postal voters made their mind up before sealing the envelope. But the Conservative lead on polling conducted by 2-3 December was variously 7 per cent (ICM), 12 per cent (Kantar), 10 per cent (ComRes) and 9 per cent (YouGov). Which to choose?

Given the relative accuracy of the 2015 and 2017 exit polls, any variance between postal voting and polling station voting was clearly not decisive. But a week before the polls, Theresa May had seemingly already haemorrhaged most of her votes. If Boris Johnson has postponed tripping over his shoelaces to within the last 48 hours of this campaign, then the exit poll will record a lower Conservative vote than transpires to be the case. So, don’t go to bed early.