Brown has 7% lead on Cameron on being a “capable PM”

The Sunday Times this morning features a Mori poll on immigration and leader ratings but without a voting intention question and without using the measures other pollsters employ to ensure that their samples are politically representative.

The most significant finding is that 47% of those surveyed think that Tony Blair should go immediately with 56% wanting him out before the end of the year. This is by far the highest rating on this question in any opinion poll and comes only a few days before the Prime Minister returns from his summer holiday.

To the question “Who do you think would make the most capable Prime Minister –Gordon Brown, David Cameron or Menzies Campbell?” these were the shares with changes on March when the question was asked last time using a similar methodology. Brown 31 (-4): Cameron 24 (-2): Campbell 11 (+2)

Regular visitors will know that whenever Mori produce a poll, whatever it is saying, I always give a health warning about the firm’s methodology.

For in all phone surveys there is a proven tendency to grossly over represent Labour because, for whatever reason, party supporters are much more likely to take part in surveys than supporters of other parties.

Thus ICM and Populus find that on average the number of poll respondents saying they voted Labour at the last election is about quarter more than actually did so and make adjustments for sample distortion. Mori don’t and their figures should be treated accordingly.

Normally with voting intention polls, which this isn’t, Mori filters its results by only including those who say they are “certain to vote” in its headline figures. So even that control does not apply here.

To give an idea of the scale of potential distortion a Mori poll for the FT just four days before the May 2005 General Election reported a Labour lead of 15%. The turnout filter cut that back to 10%. At the election Labour’s margin was 3%. Mori’s final election poll employed different methodologies and so cannot be compared.

Within the next week or so we should see the August surveys from ICM and YouGov – both of which employ robust techniques to avoid sample bias. Wait for them and ignore Mori.

In the betting on Blair’s departure the 2006 price is 10/3 with current odds of 5.4/1 on a final quarter departure.

Mike Smithson