We get this question asked every now and again. The Scottish National Party have higher GB poll ratings than the Greens, more MPs than the Lib Dems, Greens and UKIP combined, and are the third largest party in the commons - so why don't we include them when reporting Westminster VI?
The thing is SNP percentages in GB or UK-wide polls are pretty useless. Firstly, YouGov combine the SNP and Plaid Cymru into one figure meaning those are not really comparible with other polls; but whether the SNP are on 4, 5, or 6% nationally is purely down to sample variation. Indeed the only time in this parliament that they've polled outside this range (other than the combined PC percentage with YouGov) is 3% in an ICM poll in March this year. Comparing this against national voting intentions for the other parties tells you nothing about how many seats each party will win in Scotland. For this we need sub-national polling, and although this doesn't happen very often, it is much more useful in determining the likelihood of seats changing hands at a general election.
The good news is we are now tracking both Scottish and Welsh sub-national polling for Westminster on the website. Other sub-national polling (such as London and Northern Ireland) will be covered on our twitter feed.