Do the polls over-estimate Labour support?
Over at Political Betting Mike Smithson is taking John Rentoul to task for saying in his Sunday column that the polls could be overrepresenting Labour support by up to 5 points, and the Conservatives may only need a lead of 3 points to win. Mike is right to do so.
Polls back in 1992 vastly overestimated Labour. In 1997 most of them overestimated Labour, with the honourable exception of ICM, who in their final poll actually erred towards the Conservatives. In 2001 many of them still overestimated Labour, but the lessons were slowly being learnt. In 2005 all the pollsters were very close to the actual position, though the errors were still slightly in Labour's favour.
The graph below shows the difference between the actual Labour/Conservative lead at each of the last four elections, and what the last 8 published polls of the campaign showed. If there was not a systemic skew to the polls we would expect to see as many errors in favour of the Conservatives as in favour of Labour - as you can see, there is one lone instance of a eve-of-poll election underestimating Labour's position (ICM in 1997).
However, what's also clear is how much better the polls have got - in 1992 they all grossly erred in Labour's favour. By 2005 they were all very close to the real position (the one tall bar in 2005 is a MORI poll, but their final poll of the campaign was much closer, so no one disgraced themselves).
If the polling companies behaved in exactly the same way as they did in 2005 we would expect a skew in the Conservative/Labour gap of about 2 points in Labour's favour. However, they aren't behaving in the same way: to give two examples, ICM have shifted their weighting to be marginally more favourable to the Conservatives (weighting past vote to a point 75% of the way towards the real result, rather than 50% as last time); Ipsos MORI are weighting by public and private sector employment, which also shifts things in the Conservatives favour. ComRes have adopted past vote weighting. Other companies have reformed their methods in ways where it harder to say what the partisan effect will be, but no one is resting on their laurels.
On past performance and the changes made since then, I would expect the polls to be pretty close to the actual result at the next election or even, in some cases, to overestimate Conservative support. Of course, the unknown quantity is how well the polls will reflect the large shift in political support since the last election. All three of the elections since the 1992 debarcle have been Labour victories, and the pollsters have been gradually honing their techniques under those circumstances. Aside from mid term polls like European and London mayoral elections, where some pollsters have done far better than others, this will be the first time the post-1992 polling methods are tested in an environment with the Conservatives ahead.
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We should be getting ICM's monthly poll for the Guardian tonight. My laptop has thrown a wobbly, so I may not be as quick on the uptake as usual, but I will report when I get the chance.
