What went wrong

In the fullness of time I am sure much more will be said about why the polls overestimated the level of Lib Dem support at the election, but there was an interesting nugget from Andrew Cooper of Populus on More or Less on Radio 4 earlier. Populus's final poll, conducted on the Tuesday and Wednesday of election week, had the Lib Dems on 27%. However, according to Andrew in the fieldwork conducted on Tuesday the Lib Dems were in the high twenties, in the fieldwork conducted on the Wednesday they were on 24%. That looks like evidence of late swing - that the polls weren't wrong, people just changed their mind right at the end.

However, there is also some evidence that casts doubt on late swing. Because they published in the Evening Standard on Thursday and had a later deadline Ipsos MORI's final poll of the campaign had the latest fieldwork of all the pollsters - all their fieldwork was conducted on Wednesday... yet they still had the Lib Dems at 27%.

Also illustrative is Ipsos MORI's post-election poll. Most companies use some form of past vote weighting, so their post-election polls will be calibrated to the new results and won't really be directly comparable to pre-election polls. Ipsos MORI however don't use any political weighting, so their post election polls should be conducted in exactly the same way as their pre-election polls. In their post-election poll for the News of the World MORI asked how people had voted in the election on May 6th, and found figures of CON 35%, LAB 31%, LDEM 28%. No sign there of a big drop off in Lib Dem support compared to pre-election polls.

Of course - we know all about the problems of false recall, there may be people claiming to have voted Lib Dem who didn't actually do so, so this isn't conclusive either, but it isn't screaming out late swing.