Lord Ashcroft has a new batch of constituency polls out, this time looking at ten Conservative held seats with majorities of around 9% to 12% – seats that would need a Con>Lab swing of around about 5% to fall to Labour. Full details are here.

National opinion polls are currently showing a swing of around 4 percent from Conservative to Labour, so on a pure uniform swing you’d expect these seats to stay Conservative. The average swing in these polls was 3.5%, so very much in line with that. In practice though there is variation between seats, so Ashcorft did find three seats in the sample where Labour were ahead – Crewe and Nantwich, Finchley and Golders Green and Milton Keynes South. This sort of variation is inevitable, and in terms of the overall impact on seats will be counteracted by seats that Labour should be winning on a uniform swing, but where Ashcroft has found the Conservatives ahead, such as Kingswood and Blackpool North.

It is difficult to analyse the variation between seats too much – in some cases they will be genuine, and we can come up with plausible reasons to explain them (are Labour getting a bigger swing in London, for example, or is Crewe and Nantwich seeing a gradual unwind from the by-election?), but remember each constituency poll is just one poll with the normal margin of error. The fact that a poll in one seat is showing a swing that’s three percent bigger than a poll in another seat may just be normal sample variation between polls, and nothing to do with the situation on the ground.