The SNP have today released the voting intention figures from a new YouGov poll that shows the SNP ahead in both votes. The topline figures for the constituency vote, with changes from the YouGov poll the SNP commissioned back in April are CON 14% (nc), LAB 29%(-1), LDEM 16%(-4), SNP 36%(+10), Others 6%.

On the region vote the topline figures, with changes from the April poll (though if I recall correctly the SNP didn’t release the figures at the time), are CON 15%(+2), LAB 26%(+2), LDEM 15%(-4), SNP 30%(+7), GRN 8%(-3), SSP 3%(-4), Solidarity 1%(+1).

The SNP project that these figures would translate into seats as SNP 49 (+23), LAB 36 (-14), Con 16 (-2), Lib Dems 17 (nc), GRN 6 (-1), SSP 1 (-5). I’m not quite sure how they have done this, using uniform swings at a regional and constituency level I get far fewer SNP seats – only 39 to Labour’s 38 – but Scottish politics is not my forte, so I may have made an error in the distribition of the top-up seats somewhere, or the SNP may have used regional swings or a turnout adjustment. If the SNP projections are correct then Labour and the Lib Dems would not be able to command a majority in the Scottish Parliament, even alongside the Greens.

However, as some people have noted, there is also a YouGov poll in the Scottish edition of the Daily Telegraph which has contrasting figures. The Telegraph figures are CON 15%, LAB 32%, LDEM 15%, SNP 32%, GRN 4%, SSP 1% in the constituency section and CON 17%, LAB 29%, LDEM 15%, SNP 28%, GRN 8%, SSP 2% in the regional vote. On these figures Labour would remain the largest party.

So why the difference? Strictly speaking the differences could just be sample error, but I think the differences are more likely to be a result of very subtle differences in the way the polls were conducted.

As I’ve said before, all reputable pollsters will put voting intention questions at the start of a poll to make sure other questions don’t skew the answers. Neither of these polls are an exception – there were no leading questions or suchlike in the SNP poll, it was perfectly legitimate. However, what happens when you’ve got more than one voting intention question? Unlike the SNP poll the Telegraph poll included a Westminster voting intention question, and that was asked before the Scottish Parliament question. Naturally Labour had a healthy lead in the Westminister voting intentions (LAB 36% to the SNP’s 25%, CON 18%, LDEM 16%) and I suspect people were more likely to say they were going to vote Labour in the Scottish Parliament if they’d just said they would in the Westminster Parliament.

The second difference was in the options given for minor parties. The Telegraph poll used YouGov’s normal formula, giving people the choice of the main 4 parties and “other”, with people who chose other then being given a further list of minor parties. The SNP poll was slightly different – the constituency poll gave the 4 main parties and other (though with no follow up question), but the regional question listed the Greens, SSP and Solidarity along with the main 4 parties. This appears to have boosted the minor parties’ support – 15% in the SNP poll compared to 12% in the Telegraph one (which one is the better approach is a difficult question. On the face of it it is fairer to list the smaller parties along with the big four but in practice it can result in too high a score for the small parties. YouGov’s over-rating of UKIP in the last European elections, for example, was because they were listed in the main question.)

A third difference was that the SNP poll asked a separate question up front of how likely people were to vote, while the Telegraph one just gave “would not vote” as an option along with the rest. This had the potential to make a big difference, but in practice it doesn’t seem to have. The proportions of non-voters in the two polls are pretty much the same.

So which figures are “correct”? Both are fair polls and neither are outlandish enought to look like an obvious rogue. The Westminister voting intention question in the Telegraph poll might well have skewed later answers slightly, as respondents like to give consistent answers. That said, the 36% figure for SNP support in the constituency section of the SNP poll does seem quite high, and it could be an outlier. Alas, in recent years we haven’t had solid series of Scottish polling data to be able to tell when a poll seems out of line and when it doesn’t. Hopefully as we move towards next years’ elections we will get more regular polls.

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