YouGov polling made two changes to its methodology recently that have the effect of reducing Labour support in their polls by 1-2%.
1) YouGov weight their sample by Newspaper Readership (and have always done so) – but recently the number of ‘Daily Mirror’ readers in their sample has been cut sharply. YG are simply updating their readership weightings. This has the effect of reducing Labour’s support to the benefit of the other parties since about c.60% of Daily Mirror readers vote Labour.
2) YouGov used to weight all of the ‘others’ (UKIP, BNP, SNP, Plaid, Greens) together in one lump based upon the % others got at the 2010 General Election. They have changed this to weight the Scottish & Welsh nationalists separately. This, especially in a year of assembly elections, has the effect of hurting Labour support, more so in Scotland.
YG say that they had some issues in attracting satisfactory weightings of SNP voters. It is my belief that Labour’s traditional strong showing in their Scottish heartlands has been tempered somewhat since the Holyrood campaign of 2011.
A chunk of Labour’s boost in the typical YG poll used to (2010) come from their often 50%+ showings in Scotland but that has been markedly reduced as Scottish voters in many ways still think and reply to polls with their Holyrood cap on.
Libdems
It might also be mentioned that ICM polling assume that 50% of the ‘Don’t Know’ replies they receive from poll respondents will return to the party they voted for at the last election.
This, especially in the case of the Liberal Democrats has the effect of boosting them in ICM polls by 2.2% usually to the expense of Labour since voters have left yellow to the ratio of 2:1 in favour of Labour over the Tories. It is one of the reasons that Labour have always scored less high with ICM post 2010.
As Labour now hover on 40% with YouGov, just a few months ago the same polling company would have been recording 41-2% for Labour (or thereabouts) based upon the same responses from their poll respondents.