In some quarters, ICM's Martin Boon has been widely praised for his piece in Scotland on Sunday explaining the thinking behind his company's approach to "indyref" polling methodology. It was certainly an interesting insight into thought-processes that normally remain private, although as I've already mentioned I find it extremely troubling that Boon seems to be taking seriously the very speculative notion that the Yes vote is being overstated due to a 'Shy No Syndrome', but doesn't even seem to have considered the much more plausible possibility that the No vote is being overstated due to 'Shy Yes Syndrome'. More broadly, there was a certain amount of ambiguity in the article's language, which has directly led to a few daft new myths doing the rounds.
Exhibit A comes from hardcore anti-independence Tory troll Carlotta Vance (who I am now permanently banned from setting straight over at her preferred haunt, the 'Stormfront Lite' site Political Betting) -
"If the polls say 'Yes' but Scotland votes 'No' - we probably know why already."
She then goes on to quote Boon talking about "lifting the power of SNP voter voices" by weighting according to 2011 vote recall, thus implying that she has (to her relief) identified the 'innocent explanation' for the No lead having collapsed to just 3% in the latest ICM poll. Except that doesn't work. There were in fact 288 people in ICM's sample who voted SNP in 2011 - and they were weighted DOWN to count as just 238 respondents in the final results. The respondents who were significantly upweighted were non-voters from 2011 - only 181 of them were interviewed, but they counted as 309 people in the final results. Crucially, among this group there was a 10-point No lead - not huge, but enough to mean that this aspect of the weighting procedure is helping the No side, not the Yes side.
The point being of course that Boon was actually explaining how weighting by past vote recall might flatter Yes UNLESS non-voters from 2011 are also correctly weighted, which is exactly what ICM are now doing. That caveat seems to have to got lost in translation, at least as far as certain PB Tories are concerned.
What those of us who care about polling accuracy should actually be concerned about is the apparent failure of the pollsters to weight by country of birth, which we know is an extremely strong predictor of referendum vote (with those born south of the border being far more likely to vote No). The place of birth of ICM's sample is as follows -
Scotland : 75%
England : 15%
Wales : 1%
Northern Ireland : 1%
Outside UK : 8%
ICM claim in the introduction to their datasets to use targets for their weighted data that are derived from the 2011 census, which as far as I can see must mean that they are failing to weight by country of birth, because according to the census results the correct numbers for the voting-age population are as follows -
Scotland : 81.5%
England : 9.6%
Wales : 0.4%
Northern Ireland : 0.8%
Outside UK : 7.6%
So unless I'm missing something, it seems absolutely clear that a heavily No-voting section of the population has been over-represented in ICM's sample by several percentage points, meaning that even the wafer-thin 3% No lead in the headline numbers may be exaggerated.
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Note : I've updated this post with the exact census figures for over-16s, as provided by Scottish Skier in the comments section below. SS also estimates that the No lead in the ICM poll would be just 1% if weighting by country of birth had taken place.