Changes to ICM's methodology

ICM's Crewe and Nantwich poll - which I look at here - also heralds two new changes to ICM's methodology, which will probably be rolling out across their other political polls for the Guardian and Sunday Telegraph.

The first change is to to adjust their targets for past vote weighting to be slightly closer to the actual result of the 2005 election - specifically, the target is now based 80% on the 2005 election results, and 20% on the average recalled vote in ICM's polls. In theory this will produce results that are slightly better for the Conservatives and Lib Dems and slightly worse for Labour, but in practice it is a very, very minor change. Taking the Crewe and Nantwich poll, the change was not large enough to change the results by a percentage point.

The second change is more interesting. As I discussed before the London election, turnout is actually very challenging to predict and is probably one of the reasons MORI seem to have overestimated Labour's lead. One of the reasons it is so tricky is that people aren't very good at predicting their own likelihood to vote. In the UK pollsters normally rely on asking people to rate their likelihood to vote on a scale of 1-10, but this still often produces more people who are 10/10 certain to vote than actually do. ICM's new approach seems to draw some lessons from the more complex approaches taken in the USA where pollsters take into account not just people's own estimates of their likelihood to vote, but also attitudinal factors like their interest in politics, whether they see voting as a duty, and so on.

In this case ICM asked a question on attitudes to voting, asking people whether it was their duty to vote, or if people should only vote if they cared who won, or whether it was really not worth doing at all. This was then cross referenced with the 1-10 likelihood to vote scale to produce a 30 cell matrix and people were weighted by the result. This appears to be a slightly harsher likelihood to vote filter - in the case of the Crewe and Nantwich poll it increased Conservative support by 1 point and reduced Labour by 1 point - but we won't really be sure of the effect until we've seen it in action over a couple of polls.