Calculating the swing – the proportionality debate

Calculating the swing – the proportionality debate


    Baxter or Wells – which is best for Commons seat predictions?

Firstly it should be recognised that there is no fail-safe way of tapping some numbers into a computer which will then give you a relatively accurate prediction how many MPs the parties would get for those vote shares. The two most widely available online calculators employ very different mathematical approaches and can in certain instances come out with very different seat numbers.

There is one from Martin Baxter (left), who runs the excellent Electoral Calculus website. The other is from Anthony Wells (right) who operates the equally excellent and comprehensive UK Polling Report. Both are sites that I go to daily and I know both men personally.

The big division is on how the two calculate what is known as the Uniform National Swing. The Baxter approach is based broadly on the proportionality of the swing – Wells follows the traditional route of simply applying the same points change in vote shares to every seat. This is best illustrated with parties who like the Liberal Democrats at the moment are seeing their poll shares decline.

If the numbers indicate that the party has lost a third of its vote since the General Election (22.7% at the Election down to for the purpose of this example 15%) then the Baxter formula will knock a third off their share in each seat. The Wells approach would be to knock off, in this case, seven and a bit points in each seat.

    So in a seat where the LDs got 45% at the last election Baxter would reduce that to 30% while Wells has it at 38%. This can produce massive differences in seat projections.

It is important to stress that both have factored in the effect of the boundary changes that are due to come into effect before the next election. So the 2005 results that they are working with are notional rather than actual ones.

So which system is best – Wells or Baxter? The answer is inconclusive. Baxter has tested the data from both approaches from previous elections and found no appreciable differences. The strength mathematically is that going for proportionality does not produce minus numbers.

The best strategy, particularly for spread-betting on Common seats with the financial risk that entails, is to run both models and look through the seats changing hands so you can draw your own conclusions. The Liberal Democrats have a long history of holding onto seats that the national swing suggested they should lose and you need to make your own decisions.

Mike Smithson

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