So why the huge difference?

The first YouGov poll to be carried out after the “loongate” story blew up has a dramatically different picture than Survation last night.

UKIP remains on the 14% share recorded in the Sunday Times poll while the Tories edge up 2% to 31% with LAB falling back a point.

The main differences between Survation and YouGov are that the former takes likelihood to vote into account and prompts for Ukip in its main voting question.

YouGov does not ask about voting certainty – an element that has really boosted UKIP in some polls that do. For a large part of Farage’s party’s support comes from the older age groups who are far more likely to turn out at elections.

The prompt issue is one that is a matter of some debate and we know that YouGov has been testing an approach that is similar to Survation.

Currently those sampled by YouGov do not see UKIP listed on the main voting page. To select them they have to tick “some other party” which brings up another page.

It is argued that the absence of prompting depresses the UKIP share and helps boost the Tories.

Another factor that depresses YouGov UKIP shares is the firm’s party ID weightings which reduce, in this latest poll, the “value” of those who identify themselves as Ukip voted Ukip at #GE2010 by more than 75%.

How voters for “others” get their views cut back by YouGov party ID weightings.In latest poll 92 became 23. See.. twitter.com/MSmithsonPB/st…

— Mike Smithson (@MSmithsonPB) May 21, 2013

Mike Smithson

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