



At this point it should be clear that the basic assumption that party support changes roughly evenly across the country is wrong, and so claims like 'this new poll says the Conservatives are ahead in Perth & North Perthshire' are a bit of a stretch.





If I've already convinced you, great, this was the message I was trying to get across. But if you want to keep reading, here's some more evidence.





Seat calculators usually get updated quite quickly after elections with new data. It is easy though to go back over previous elections to check what the prediction would have been using this technique had you entered the actual national vote totals rather than a poll.





In both 2015 and 2010 the result is broadly accurate. But 2010 was a 'no change' election, with no seats changing hands and vote totals barely changing. 2015 was the complete opposite - a wave election where one party ran away with all the seats.





The chances are 2017 will be neither of these. The current polling is an SNP to Conservative swing of 10-12%. This is not to say that the polls are accurate, or that that is what the eventual result will be. That is simply what public polls are saying right now. And that is somewhere between the Scotland-wide stasis of 2010 and the 24% wave of 2015. So how to solve the problem? Holyrood.





In 2003, 2007 and 2016 elections were fought on the previous boundaries and had changes in support for various parties in the constituency vote that lay between these two extremes.





In each case the UNS technique would have been within a few seats of getting the totals right, but when it comes to specific seats, is much less impressive.









The top left box is where the UNS method gets it right. The bottom left has the 'surprise' seats, the top right the dogs that didn't bark, and the bottom right the rest.













By and large then, it's possible to extrapolate roughly a poll to total seats, but in each of these three elections there were plenty of individual seat results that UNS wouldn't have predicted - usually as many or more than the changes it successfully predicts.





Calculating that Labour would hold Glasgow Pollok in 2003 and the SNP would win Angus in 2007 doesn't need fancy maths. The test is in the battlegrounds. And when it comes to predicting which seats will actually change hands the technique that gets spraffed across the media (mainstream and online both) just doesn't work.

Polls are great, aren't they? Elections are a contest, and until there is a final score there is a natural human interest in a running tally. In the end though, votes don't necessarily matter as much as seats. Just ask the 1.2m people across the UK who voted Green in 2015 only to elect one Member of Parliament.Without a fair voting system, it's natural for the first interpretation of any poll to be to as which seats will change hands. People plug the figures into calculators like Scotland Votes and get a lovely graphical map of what Scotland now looks like. It's got maths so it must be right, eh? Stuff like this is also common:The problem is that the way this is calculated just plain doesn't work.The technique that is used - Uniform National Swing (UNS) - iseffective writ large. It takes the nationwide change in party support and just applies it in each constituency. Fair enough. But this is what the local swings actually looked like in Scotland in 2015. They weren't uniform. They varied. Substantially.The national swing from Labour to SNP was 24%. 14 constituencies were within 2% of that. The rest were considerably above or below. Some were right out there at the extremes. On election night 2015 Glasgow North East was so far from the average that it famously 'broke' the BBC swingometer graphic.