How did uniform national swing (UNS) do in 2010?

The question of whether or not uniform national swing (UNS) calculations are a sensible way of trying to turn national opinion poll vote figures into seat estimates has been much debated in the last few months. So how did UNS do this time round?

Here is how the May 2010 general election result compares with a UNS projection based on the actual vote changes which occurred between 2005 (notional results) and this time:

Conservatives: 305 seats*. UNS prediction: 291 (-14)
Labour: 258 seats. UNS: 266 (+8)
Liberal Democrats: 57 seats. UNS: 62 (+5)

* Excluding Thirsk & Malton from calculations

Uniform swing versus proportional swing: which is best?

Presenting a new analysis of the merits of the two main ways of converting party vote shares in to seat number projections... more

In a close election the errors between UNS and the actual result come with political significance. Under UNS the vote shares the parties secured would have given a Parliament where the combined numbers of Labour and Liberal Democrats was 328 rather than 315. That 13 seat difference could have made hung Parliament negotiations very different.

That said, the overall record of UNS is far better than you might guess from some of the comments made about it in the run-up to polling day. UNS didn’t turn in a perfect prediction but it got pretty close. The biggest error – on the Conservative Party’s seats – was 14 seats. After all the talk about Ashcroft money, difference performances in the marginals and the like that is more a mouse than a mountain.

Whilst it’s certainly true that UNS based predictions are sometimes quoted as if they are certain to be correct – and that’s wrong, they should have a health warning – you’re probably more likely to have been misled by people saying, “Oh, the result will be nothing like UNS because of this long set of reasons…” than by someone relying on UNS.

UNS also had a pretty decent record in both 2001 and 2005. Prior to this election it was a fair question to ask whether a change of government would see, as happened in 1997, uniform national swing projections turn in a much less accurate prediction. As it was, that didn’t happen and it is UNS’s poor performance in 1997 that looks to be the exception:

2005 general election

Conservatives: 197 seats. UNS prediction: 184 (-13)
Labour: 355 seats. UNS: 369 (+14)
Liberal Democrat: 62 seats. UNS: 62 (+/-0)

2001 general election

Conservatives: 166 seats. UNS prediction: 181 (+15)
Labour: 412 seats. UNS: 402 (-10)
Liberal Democrat: 52 seats. UNS: 47 (-5)

1997 general election

Conservatives: 165 seats. UNS prediction: 207 (+42)
Labour: 418 seats. UNS: 395 (-23)
Liberal Democrat: 46 seats. UNS: 28 (-18)

UPDATE: What does this mean for the Liberal Democrat prospects in 2015? A lot depends on how much 1997 can be repeated (note the 18 seat improvement on UNS then). For more on that and the likely range of options, see edition 40 of Liberal Democrat Newswire.

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