The Green Party have just released an ICM poll of Brighton Pavilion that shows them leading the Conservatives by 8 points in the seat with Labour in third place.

Clearly the Greens have commissioned it for their own purposes, and one should always look carefully at polls commissioned by political parties – they aren’t releasing those figures out of the goodness of their hearts – but in this case it appears kosher.

The full figures, with changes from the notional figures at the last electon, are CON 27%(+4), LAB 25%(-13), LDEM 11%(-5), GRN 35%(+14).

I need to add two caveats – firstly the sample size was only 533. Rather counterintuitively, just because you are polling a much smaller population than a poll of the whole country, it makes virtually no difference to the sample size you need for a given margin of error, so the margin of error on the poll once you exclude don’t knows and won’t votes is going to be somewhere in the region of 5 percent.

Secondly, there is the question asked: “Labour, the Conservatives, the Green Party, the Liberal Democrats and other parties will fight a new election in 2010 in your area. If there were a general election tomorrow which party do you think you would vote for?” Normally pollsters do not prompt by minor parties in voting intention questions, while this doesn’t seem very fair, it’s what years of experience suggest gives the most accurate answer. However, here we have a rather unusual situation where a minor party are obviously at least in the running to win the seat given the last general election and their strength on the local council, what’s more local voters are reasonably likely to be aware of it. While including the Greens in the prompt will probably have boosted their support, it would have been perverse not to given the situation in Brighton Pavilon.

After the wild moves in Ipsos MORI’s last two polls, which showed the Tory lead tightening to 6 points and then spiking back to 17, we’ve almost the mirror image from ComRes. Their last poll showed a 17 point, and tomorrow they have a new poll in the Independent that shows a sharp reduction. The topline figures, with changes from their poll just over a week ago, are CON 38%(-3), LAB 29%(+5), LDEM 19%(-2).

Whereas the big movements with MORI were down to the lack of political weighting allowing a sample with a very perculiar amount of 2005 Labour voters, with ComRes my guess it is the rather more mundane explanation of a rogue poll – in hindsight the 17 point Tory lead in their last poll looks wholly anomolous, the only other pollster showing such a low Labour share of the vote was Angus Reid, who seem to consistently show a lower level of Labour support for methodological reasons. If we put ComRes’s previous poll to one side and look at the one prior to that, the shifts are far smaller, with the Conservatives up, Labour up 2 and the Lib Dems down 1 – no significant movement in itself, but chiming with the recent slight strengthening for Labour.

So, as we head to the end of the year (YouGov/Telegraph is still outstanding in theory, but I’m not sure when it will arrive), we still have quite a broad range of polling figures, with leads between 9 and 17 points – from a hung Parliament to a Tory landslide. The Conservatives are in the range 38%-43%, but of the 12 polls in December 9 have put them at 40% or 41% – that’s a noticable difference from November when 7 out of 10 polls had them below 40%. Lib Dem support ranges between 16% and 21%, but mostly between 18% and 20%. The real variation is in the level of support pollsters are finding in Labour’s support, from 23% to 31%. However, the lower figures there are either AngusReid or that single ComRes that appears to have been a rogue, and other figures are in the tighter range of 26% to 31%. That leaves us with an average lead of around 11 points or so – on a UNS right on the cusp of a hung Parliament, though in practice it would probably result in a Conservative majority.

(As an aside, if you are following the polls in the glorious rumourmill of Twitter then the 3 point Tory lead in MORI on Sunday has been followed by CON 40%, LAB 31% in this poll. Boy, are polls in the general election are going to be fun there!)

The full tables for MORI’s poll are now available here.

A notable finding there is that economic optimism has significantly dipped since the PBR, down from plus 10 last month to minus 4 now. I think this is the first poll on economic optimism since the PBR, but is entirely in line with the YouGov/Sunday Times poll that showed people becoming much more negative about the country’s current economic situation after the PBR.

Turning to voting intention, there is a shift in propensity to vote, 68% of Tory voters now say they will definitely vote, compared to 61% last time. The proportion of Labour voters who say they will actually vote is down 2 points to 49%.

However, the biggest difference between this month and last is the make up of the sample itself. The past vote breakdown in this poll was CON 22%, LAB 28%, LDEM 9% (the equivalent, if you exclude did not votes and so on, of CON 33%, LAB 43%, LDEM 14%). Compare this to last month’s past vote breakdown of CON 19%, LAB 30%, LDEM 10% (the equivalent of CON 29%, LAB 46%, LDEM 16%). It is the lack of political weighting that has produced such extreme switches in support in MORI’s recent polls – ICM last weekend had a similarly pro-Labour sample, but their weighting brought it back to showing a much smaller swing.

MORI do not weight by past vote because of false recall. The question is not whether false recall exists or not (all the established pollsters accept that), but how variable it is. ICM and Populus believe it is generally steady and doesn’t change wildly from month to month (the implication is that it is largely the result of a social desirability bias making people claim to have voted when they didn’t, and the way people report tactical voting). MORI believe that it does genuinely vary slightly from month to month, and that there is the potential for it to change significantly (the implication there being that it changes with people’s current political preferences).

Politically weighting polls makes them less volatile. However, MORI’s contention would be that the public are genuinely very volatile, and that companies like ICM and Populus risk weighting out genuine volatility. There’s no way of proving with certainty who is correct, though one thing to look at is the results of YouGov, who take a notably different approach. YouGov weight by party ID rather than past vote, but more importantly, they weight using information given by their panellists in the past (either on election day 2005 or when they joined the panel), rather than using data collected as part of that day’s survey. So, while YouGov’s weighting targets could be wrong, they can at least be confident that they are not changing from day to day. Their method would not dampen down genuine volatility, and their figures are one of the less volatile.

For those of you not bored to tears by discussion of methodology there is, incidentally, a one-day conference at the Royal British Academy next month (jointly hosted by the British Polling Council and the National Centre for Research Methods) discussing approaches to polling methodology, featuring Peter Kellner, Andrew Cooper, Martin Boon, Nick Moon and – most relevantly to this, John Curtice discussing whether it is safe to weight by past vote (Simon Atkinson of MORI is also there to put what I expect will be the counter argument!). More info here.