Beware of outliers – mixed poll results indicate dead heat

Last week, three polls in a single day showed clear Labour leads – bringing the Guardian to declare on its Friday front page that it was “The day the polls turned”:

The copy on the front page took a more measured take on the difficult art science of interpreting polls, as did the write up on LabourList – especially after two polls released later on Thursday evening both showed a Conservative lead.

Today, a flurry of new polls have again commanded attention. As is often the case, it is the most surprising one which has received the most attention: ICM/Guardian have a six point lead for a Conservatives, on 39%.

However, there have been three other polls showing contradicting results.

Last night’s YouGov put Labour in a three point lead:

Labour – 36%, Conservative – 33%, UKIP – 13%, Lib Dem – 7%, Green – 5%

While this morning, Populus saw the two main parties tied:

Labour – 33%, Conservative – 33%, UKIP – 15%, Lib Dem – 8%, Green – 5%

Today’s Lord Ashcroft shows a similar outcome:

Labour – 33%, Conservative – 33%, Lib Dem – 9%, UKIP – 13%, Green – 6%

And this afternoon’s ICM gave a boost for the Tories:

Labour – 33%, Conservative – 39%, Lib Dem – 8%, UKIP – 7%, Green – 7%

Overall, this looks unclear, but points to Labour and the Conservatives polling roughly similar number – it’s worth noting that they are both on 33% in three polls each.

As the biggest break from the norm, the ICM poll is the likeliest outlier (which is exactly what we said about the Panelbase poll that showed a six-point Labour lead last week).

There has already been some analysis of why ICM may have produced the result it did: Martin Boon from ICM Unlimited said the raw data “could be a just touch too Tory”. Certainly, the breakdowns suggest this could be the case. Those bracketed as C2DE (people on lower incomes) are polled as favouring the Tories 41% to Labour’s 28%. This is an almost exact reversal of what we usually see from people in this group. The sub-samples also indicate big leads for the Conservative Party in the North and Scotland.

While Labour’s 33% in ICM is roughly the average for the party, the Conservatives 39% and UKIP’s 7% are way off the norm.

The Guardian have also published an article noting the differences between phone and internet poll results (ICM is a phone-only poll).