Based on the concept of a pendulum – not a ratchet

Later today we’ll be introducing a totally new commons seat calculator that has been developed by long-standing PBer and statistician, Andy Cooke. This will be published here sometime after lunch.

It’s stems from the thinking outlined in Andy’s initial post yesterday in which he argued for the merits of “pendulum thinking” as we seek to project the coming election rather than the mind-set of “the ratchet.”

For the conventional approach to seat projection is simply to go back to what happened in May 2005 without any thought of the previous two elections when there were disproportionated swings in what for the coming contest are the key marginals.

What happened before is locked in as if in a ratchet. Andy wants us to think in pendulum terms and go back to Tony Blair’s landslide in 1997.

The percentage movements are huge. Andy wrote: “….when all three elections are taken into account, the 2010 Labour seats with up to 10% majorities hold an extra pro-Labour swing of 4.3-4.9%. This drops off to 4.2% for the marginals with majorities of 10-15% and 2.7% for those with 15-20% majorities, and drops down to negligible amounts beyond that.”

So what happens if a proportion of this historical swing starts to unravel – a critical question because so much of it occurred in the constituencies which are the 2010 key battle-grounds and where YouGov, ICM and Ipsos-MORI have recorded disproportionate moves.

Andy’s post this afternoon sets out the detailed assumptions and illustrates them with a table showing the effect at different levels of Tory lead/deficit. We get a range of possible seats for the three main parties together with a percentage probability of three separate outcomes – Conservative majority, Conservatives the largest party, Labour the largest party.

We also get what will be a very controversial suggestion of what the minimum vote threshold for a Tory majority government might be.

The reason we are publishing this in three separate segments is partly for our own convenience – I’m out all day and Andy won’t be able to deal comments until he gets home from work – and partly to prepare the ground. Those who have not yet read Andy’s post of yesterday morning might like to do so.

Only time will tell whether Andy is right or not but at the very minimum his challenge to the prevailing consensus will lead to a more informed approach to the coming battle.

Mike Smithson