The Problem with Swingometers
John Lanchester · 72 Hours to Go
So, 72 hours to go, and the polls are pointing with a quavering, accusatory finger at a hung Parliament. Or maybe it’s a confident, optimistic finger – anyway, that’s where they’re pointing. YouGov has the Tory/Lab/Lib Dem projection as 34/28/29, ICM has it as 33/28/28. UK Polling Report looked at all five of the polls published on Sunday, four of which showed some momentum for the Tories after the third debate, and all of which showed the Tories as falling short, with the lowest projection 264 seats and the highest 315 (the target, remember, is 326).
Those estimates assume something called a Uniform National Swing, i.e. that the vote will move by the same amount everywhere. This is probably an unsafe assumption, of the necessary-evil kind which always needs to be made in building a mathematical model. On UNS, the Tories need a 6.8 per cent swing away from Labour. But the Tories are, obviously, concentrating their efforts in marginal seats, which complicates things, as does the surge in support for the Lib Dems, which will hurt Labour in some places but will hurt the Tories in others; and in the last three elections the voters have shown a big propensity for anti-Tory tactical voting. If that changes then the electoral map will change too. For those of you who are far gone in poll nerdery, there is a fascinating, detailed and very technical analysis by Nate Silver here.
Silver is the US pollster whose site played a big role in analysing and explaining the process of polling during the presidential election. He says that ‘the science of UK electoral forecasting is not terribly advanced’, and does a good job of explaining just why it’s so complicated. He rejects UNS and has a different way of doing it which I’m not going to attempt to summarise. His model has the Tories on 299 and the Lib Dems on 120, ahead of Labour on votes but behind on seats.
The spread-betting markets, which have historically been more accurate than polls, are far more bullish about the Tories, and show them as clocking in at 323-328 seats: in other words, the middle of the spread is bang on the target for a majority. If Silver is right, you can make some money betting on the Lib Dems, who according the the spread are on 82-86 seats. Put this all together, and it fits the definition of ‘too close to call’.