This weekend the Electoral Reform Society added to the worries around the Police and Crime Commissioner election when they published an estimated turnout of 18.5% for November's election. While Policing Minister Nick Herbert had famously spent an earlier part of the week on the Today programme refusing to give Evan Davis a figure for what would constitute a good turnout, it is unlikely that the Society's prediction will have brought him much cheer.

However, it may be worth looking a little deeper, at the Society's own document on how they arrived at their figures.

Firstly, they started with a baseline of 'c. 34%' because 'Recent Local Election Turnouts are in this region'. I don't disagree with that, or with the decision to assume a turnout baseline similar to a local election rather than a national one, but being used to reading opinion polls and research reports, the description of the baseline was lacking a certain precision that I have come to expect.

They deduct 6% because the election is in November rather than May, based on data from Rallings and Thrasher on variable turnout at local by-elections – seems fairly sensible. Rallings and Thrasher are acknowledged good-eggs/experts in this sort of thing.

Then they deduct 5.5% to reflect the fact that turnout is lower when there is no free mailshot, and boosted when there is one.

Then they deduct 4% because there aren't going to be any Party Political Broadcasts (technically Party Election Broadcasts, but I'll not fuss too much about that).

But, WAIT A COTTON-PICKIN MINUTE, did they really just deduct 5.5% for there being no free mailshot from a local election baseline? Local Elections don't have free mailshots, so surely the lack of one is factored in already in the 34% they started with? Taking it off again is a form of double-counting.

Now, there are other factors they mention but couldn't measure, and others they perhaps didn't mention, but I noticed that while my references to their 18.5% turnout were retweeted, my query as to the double-counting did not fare so well. Sometimes we are not truly happy unless we are being grumpy.

Update: – the Electoral Reform Society have responded to this article in the comments section of an article on their website. They explain the reasons behind 34% (which is fine), and say the 5.5% deduction is about measuring campaign intensity rather than the presence of a mailshot per se. Thanks to them for the response – have a look everyone and see what you think.