ICM’s monthly poll for the Guardian yesterday had the Conservative lead up to 5 points. This is the largest Conservative lead in an ICM poll since prior to September 1992. The topline figures, with changes from last month, are CON 37% (-1), LAB 32% (-2), LDEM 21%(+1) and suggest that Labour have not yet begun to recover from the problems that have recently beset them. It also shows no decrease in the level of support of “other” parties.

Interestingly ICM’s adjustment for the spiral of silence had a large effect upon these figures. After the failure of the polls at the 1992 election, one of the explanations put forward for the errors was that of “Shy Tories” – the Conservative party had become so tainted in people’s eyes that they were embarrassed to admit to pollsters that they were going to vote Tory and instead refused to answer or claimed they didn’t know how they would vote. To deal with this ICM started adjusting their published figures by assuming that half of those who said “don’t know” would actually end up voting for the same party they told ICM they voted for at the previous election – the accuracy of this adjustment has subsequently been supported by several studies that recontacted “don’t knows” after elections to see how they actually voted.

Initially the adjustment tended to boost the reported level of Conservative support in polls and was one of the reasons why ICM was the most accurate pollster in 1997. Sometime around 2000 the balance slowly shifted and ICM’s adjustment started to favour the Labour party, presumably because people were starting to be more embarrassed to admit to voting Labour (the phenomenon was dubbed “Bashful Blairites” by Andrew Cooper of Populus). In this poll the adjustment has reduced the Conservative lead by 3 points; the unadjusted figures were CON 39%, LAB 31%, LDEM 21%, but ICM has estimated that many of the people who said “don’t know” would vote Labour if there was really a general election tomorrow, cutting the lead to 5 points.

This adjustment is likely to be part of the reason why MORI, who make no such adjustment, are reporting larger Conservative leads than the other pollsters (Populus make a similar adjustment to ICM, and it is thought that people are not so embarrassed to admit to unfashionable opinions on internet polls like YouGov’s).

As yet the voting intention figures are the only ones available – the Guardian do sometimes stretch coverage of their ICM polls over several days so something else may yet appear.

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