Polls stability again (well, sort of)

Again, the polls appear to be all over the place. Last night’s ComRes poll had the Conservatives in front by nine points, Populus by just one. You Gov had the Lib Dems ahead by three, Angus Reid had them ahead by just one. Lib Dem support had either ‘burst’ (ComRes had them back to 26%, albeit a level of support that many Lib Dems would have sold their souls for just a week ago) or was at its highest level (34%, with YouGov).

There’s no doubt that there’s some fluctuations. But, as with the pre-debate polls, there’s also some real continuity, once you take into account the margin of error you get from sampling.

The Conservative vote is the most stable of all. Since the Lib Dem surge, every poll has put the Conservatives on 33+/-2.

With one exception – yesterday’s Angus Reid poll – every poll has put Labour on 27+/-3.

And with two exceptions – yesterday’s ComRes and YouGov polls – poll survey has put the Lib Dems on 30+/-3.

And remember that each polling company conducts their surveys differently – different ways of doing fieldwork, and then (even more importantly) different ways of weighting and filtering the data – and so we should expect to see variation between companies.

If however we look at the spreads by companies, then we see even more stability. Excluding those organisations which have only done one poll since the Lib Dem surge:

ComRes have: Con: 33+/-2; Lab: 27+/-1; LD: 28+/-2.

YouGov have: Con: 32+/-1; Lab: 28+/-2; LD: 32+/-3.

Angus Reid have: Con: 32+/-0; Lab: 24+/-1; LD: 33+/-1.

ICM have: Con: 34+/-1; Lab: 20+/-1; LD: 29+/-2.

In other words, within polling companies, not a single poll has seen movement outside the margin of error, and the Conservative and Labour spreads are even more stable, the majority being just +/-1.

One other thing. The +/-3 margin of error is 95% accurate, but that means that one in every 20 polls will show variations beyond it. The phrase rogue polls is used wildly – as Mike Smithson of politicalbetting.com often says “a rogue poll is one whose results you disagree with” – but it technically refers just to those 5% of polls.

We are now seeing so many polls – 16 since the debates alone – that we should expect about one poll every five or six days to be a rogue.

UPDATE: This piece was written before the publication of today's Ipsos-MORI poll, but that merely confirms the argument above, with the poll nestling nicely near the mid-points, at 32/28/32.


Professor Philip Cowley