ComRes/Newsnight poll has AV and FTPT equal

There is a new ComRes poll on the Alternative Vote for Newsnight tonight, which - in response to the question on the ballot paper - has YES and NO equal on 41%. This is in contrast to other polls asking the bare referendum question, which have tended to show AV ahead (indeed, the most recent ComRes poll conducted three days before this one had a ten point lead for AV).

What's the reason for the difference? My guess is that it is because in this poll the referendum question appears to have been prefaced by four statements giving some pro- and anti- arguments that are likely to be used in the campaign, and perhaps exposure to these moved people against AV. Certainly, the proportion of don't knows was much lower than ComRes normally find, and the NO statements used met with more agreement than the YES ones - 65% agreed that the system of electing MPs needed overhauling, but only 38% agreed that having more coalition governments would be good. 63% agreed the referendum was a waste of money, 81% agreed that people's first preference votes should count more than other people's 2nd, 3rd or 4th preferences.

It's also possible that the difference is because this was a phone poll, rather than online, or perhaps because the poll wasn't politically weighted (ComRes often don't use political weighting on polls without voting intention questions, though I'm not certain on this one), but my guess is that it is due to the statements prefacing it.

Of course, theoretically it's possible that loads of don't knows made their mind up to vote NO at the end of last week, but I suspect that isn't the case. Note that the poll was conducted over the weekend, so before the NO campaign's launch this week and their focus upon the cost of the referendum.