Here are some excerpts:
.... All of the companies who released polls within a few days of the general election overstated the level of Lib Dem support, by between 2 and 5 points, but the reasons are still unclear, and will probably remain so until pollsters get to test their methods against the next general election. Basically the possible reasons boil down to reasons of people changing their intentions, pollsters misjudging their intentions, or pollsters polling unrepresentative groups of people.I highly recommend you go over and take a look for yourself, as it also speaks a lot towards some of the methods that pollsters here in Canada use. Given that in the last few days of the 2008, the majority of pollsters were a bit off as well (some also overestimated NDP support), it's interesting to find the correlation in numbers, not to mention that some of the companies over there are the same ones we have here, more or less.
.... Final polls strongly suggested that the Lib Dem poll was soft – several companies included questions on whether people might still change their mind and found Lib Dem voters were still unsure. However, just because people are uncertain doesn’t mean they necessarily will change their mind. If there had been a late swing away from the Lib Dems, then it should have been picked up by recontact surveys after the election. Angus Reid, YouGov and ICM all recontacted people who were polled late in the campaign to see if people who told them they would vote Lib Dem changed their minds after the final polls – all found neglible levels of late swing.
... Clearly the reason for the error could have been don’t knows breaking disproportionately for Labour and Conservatives. ICM’s recontact survey found don’t knows broke disproportionately for Labour and their topline adjustment made their figures more accurate, MORI’s squeeze question also boosted Labour – so it would seem the pollsters got this one right. YouGov don’t use any reallocation of don’t knows – but their re-contact survey showed don’t knows splitted pretty evenly (there are likely to be different patterns of saying don’t know when there is no human interviewer).
Also - for those that read this blog and are interested in UK politics, to the right I stole another thing off of UK Polling Report, and added YouGov's daily poll widget. Easy enough to figure out: "C0n" is the governing Conservative Party led by PM David Cameron; "Lab" is the Opposition Labour Party, led by Ed Milliband, who I've talked about here; "Lib" is the Liberal Democrats, led by Nick Clegg and in a governing coalition with the Conservatives; and "Oth" is the "other" parties, which include the Northern Ireland parties, the Celtic nationalist parties, and others like the Green Party and UKIP.
If anyone knows of a similar widget for Canadian politics, I'd love to have that up as well!