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How valid is the UNS in the current context?

April 19th, 2010

Could this affect Commons seats projections?

Most people with an interest in politics and election outcomes are familiar with the term – Uniform National Swing but there is often a lot of confusion on how the numbers are made up?

For a key element is that the swing is not applied proportionately in each seat – but according to the number in percentage points change that, say, a new poll represents.

So based on the latest 33% poll share the Lib Dems are up by nearly half again on the 22.7% of last time. But when the UNS calculation is applied it is the 10.3 point difference that is added on to what happened in each seat in 2005.

So in a seat where the Lib Dems got 30% in 2005 the UNS approach would give them 40.3% now. Applying a proportional swing would make that just under 45%.

Whether that is right or wrong I do not know but at the margin this could make all the difference in determining whether or not there is a projected gain.

I think I’m correct in saying that the UNS approach has never been tested in circumstances where one of the main parties has experienced an uplift on the scale we have seen in the past couple of days with the Lib Dems.

At the moment we are in uncharted territory and you punt at your peril.

Mike Smithson