In the last post I discussed the ongoing disparity between TNS-BMRB, which currently shows a Yes vote of 40% after undecideds are excluded, and the three Yes-friendly online pollsters that are currently clustering around a pro-independence vote of 45-47%. I wondered to what extent the difference is caused by the fact that TNS are not reliant on volunteer online panels, and to what extent it's caused by them applying an eccentric weighting procedure. (They scale up people who didn't vote in 2011 or can't remember how they voted so that they count as a full 50% of the sample, which is a much, much higher figure than any other pollster uses.) So in order to test this important question, I thought it might be useful to compare the TNS cross-breaks with those of Survation and Panelbase. I'd like to have included ICM as well, but those datasets haven't appeared yet.
In order to make a meaningful comparison, it's necessary to strip out the Don't Knows, because TNS always report a far higher number of undecided respondents.
PROPORTION OF 2011 SNP VOTERS WHO SAY THEY WILL VOTE YES :
TNS-BMRB : 79.6%
Panelbase : 78.7%
Survation : 82.0%
PROPORTION OF 2011 LABOUR VOTERS WHO SAY THEY WILL VOTE YES :
TNS-BMRB : 17.6%
Panelbase : 32.3%
Survation : 27.9%
PROPORTION OF 2011 CONSERVATIVE VOTERS WHO SAY THEY WILL VOTE YES :
TNS-BMRB : 6.3%
Panelbase : 8.2%
Survation : 7.8%
PROPORTION OF 2011 LIBERAL DEMOCRAT VOTERS WHO SAY THEY WILL VOTE YES :
TNS-BMRB : 17.1%
Panelbase : 27.3%
Survation : 11.3%
So there's good news and bad news here. The bad news is that the bizarre TNS weighting procedure plainly isn't the only explanation for the disparity - they really are finding a significantly lower number of Labour voters 'on the ground' who say they will vote Yes, and that's clearly having an impact on the overall numbers. That would be the case even if they weighted their raw data in exactly the same way as Panelbase and Survation.
But what I find fascinating is that TNS are showing an identical position to the online pollsters in respect of people who voted SNP in 2011. This goes completely against the conventional wisdom about why online polls might conceivably be skewed towards Yes - ie. that highly opinionated, die-hard nationalists are signing up to online polling panels in disproportionate numbers. If that was the explanation for the higher Yes vote, you'd expect the difference to show up in the SNP column, not the Labour column. Why would the supposedly rogue element in the online polls - the surplus numbers of opinionated nationalists - have voted Labour in 2011? It doesn't make any sense, so clearly there is something more subtle and interesting going on.
The solution to the polling puzzle in this campaign therefore seems to hinge to a large extent on understanding the greater tendency of online pollsters to pick up a substantial Yes vote among Labour (and possibly Liberal Democrat) voters. Could it be that such people feel a bit more embarrassed about admitting their support for independence, due to their political backgrounds? Do they find it easier to be honest to a computer screen than to a real live interviewer? That's just one possible explanation out of many (another is that Labour voters who use the internet more often are likely to be disproportionately well-informed about the issues), but if it's the right one it would be fantastic news for the Yes campaign, because it would suggest that the online polls may be considerably more accurate.