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The life of an MP in a marginal seat is not a glamorous one. By far the most important thing you can do as an MP is to use your status as a local champion in order to win over future voters.

That means sorting out people's housing problems, giving people tours of the Houses of Parliament, meticulously replying to correspondence, campaigning in the rain on Saturday mornings…

This is not House of Cards politics. It’s grinding, hands dirty, sleeves up, usually thankless hard work.

But for each signed letter, or school visit, every media appearance and surgery appointment, a pattern will emerge. More people will see you, recognise you’re one of the good guys and vote for you come polling day.

This is the incumbency effect, and you won’t understand the results on Thursday unless you’re au fait with it.

Almost all of the seats that Labour is looking to take off the Conservatives on Thursday were won by the Tories in 2010.

That means, in pretty much every Labour target, they are going to have to deal with the electoral dividend that comes with being an MP.

How much of an advantage is this to the Conservatives? Well, Chris Hanretty (otherwise known as the personification of the Newsnight Index) has crunched the numbers.

He found that Conservative MPs who are running second time get 2% more than Conservative candidates who have ‘inherited’ their seat :

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Curiously Labour has a slightly smaller incumbency lift it seems – mainly because their fresh candidates do better (relatively) than Tory ones.

The really interesting figures here are the Lib Dem stats. A sitting MP is worth nearly four points more than a fresh MP and a whopping 7% more than the national figure. It is this Liberal Democat ground success that leads many in the party to believe that they will have a surprisingly reasonable night on Thursday, holding seats they should by rights have no expectation of retaining.

The problem for the Lib Dems is the Newsnight Index factors in incumbency – and it still looks pretty grim for them.

So, for all you political geeks out there, here are the unbalanced figures – what the predictions would be if there was no incumbency factor.

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They show that Labour would be easing ahead in the polls without the incumbency effect – nearly 25 MPs ahead of the Tories. Instead, they’re currently forecast to get fourteen fewer MPs than the Conservatives.

The figures also show the Liberal Democrats – without their stellar local campaigning -would be on a paltry 15 seats come Friday morning. They would lose nearly three quarters of their MPs.

The quality of MPs is also hugely variable, complicating attempts to identify an incumbency effect. Some first time MPs will see barely any electoral disbursement from their five years in office. Others will see a much larger bounce. This presents pollsters with a psephological nightmare – the uncertainty of local campaigning. It’s why the constituency based Ashcroft polling is so important.

There is an added problem for psephologists. There are former MPs, unseated in 2010, who are running in the same seats this time. Seats like Camborne, Redruth and Hayle, where the Lib Dem’s Julia Goldsworthy is standing. Or Labour’s Bob Blizzard in Waveney. They can potentially neutralise the incumbency effect by simply being known in their respective constituencies.

Finally the incumbency effect – or lack of it – goes a long way to explain how UKIP could pick up seats on Thursday. Many of their targets do not have an incumbent MP. Grimsby, Boston and Skegness, South Thanet – not to mention their two by-election victories that by definition had no sitting MP.

Allegra and I are in Stockton South today - which has a nominal 1% lead for the Tory incumbent James Wharton. It’s very likely that if he holds the seat it will be because of his incumbency.