So what happened to the bashful Brownites?

Everyone will know the result of the Crewe and Nantwich by-election by now, but what about the polls during the campaign, how well did they do? By-election polls have been a rare creature in recent years, but such was the attention paid to this contest that we saw three of them, two from ICM and one from ComRes.

All of them showed the Conservatives in the lead, so no pollsters looked silly this morning, but apart from having the right party in the lead they were actually a long way from the result.

ICM/Mail on Sunday (May 8th) - CON 43%, LAB 39%, LDEM 16% ICM/News of the World (May 16th) - CON 45%, LAB 37%, LDEM 14%

ComRes/Independent (May 18th) - CON 48%, LAB 35%, LDEM 12% Result (May 22nd) - CON 49.5%, LAB 30.6%, LDEM 14.6%

ComRes were very close to the actual level of support for the Conservatives, but everybody overestimated the level of support Labour would actually get, and ICM especially were well short of the actual 19 point Tory lead.

Firstly we should add a caveat that all the polls were done several days before polling day - ICM's last poll was 6 days previous, leaving 6 days for people to change their minds. Given the nature of by-election it's perfectly possible the electorate swung even further behind the Tories in those final days (such was its brevity, 6 days was a quarter of the whole campaign!).

I suspect the actual reason was the re-allocation of don't knows that both ICM and ComRes did. ICM assumed that 50% of people who said they didn't know how they would vote would end up voting for the party that they voted for in 2005, ComRes reallocated all their don't knows to the party they voted for last time. Without those adjustments their figures would have been:

ICM/Mail on Sunday (May 8th) - CON 51%, LAB 30%, LDEM 15% ICM/News of the World (May 16th) - CON 49%, LAB 34%, LDEM 13%

ComRes/Independent (May 18th) - CON 49%, LAB 34%, LDEM 12% Result (May 22nd) - CON 49.5%, LAB 30.6%, LDEM 14.6%

Which are all far closer to the actual result than the adjusted figures were, it looks as though all those Bashful Brownites that ICM and ComRes allowed for never turned up at the polling stations.

ICM's re-allocation of don't knows is based on solid research from past elections that shows don't knows do tend to break in favour of the party they've supported in the past, so I don't intend this to be a criticism of their approach to general election polling. It just appears that it doesn't work when it comes to a by-election.