Calculating Swings

A question that occasionally comes up in the comments here is how exactly you calculate a swing in an election. It obviously needs answering since the BBC managed not only to get it wrong on their coverage last night, but still had it wrong on the Today Programme this morning when they claimed there was an 8% swing to the SNP.

To calculate a swing, take the increase of the vote for the party you are calculating the swing to, then subtract the swing to the party you are calculating the swing from (if that party's vote has gone down, then this means you will be adding it). Finally divide the total by two.

So, in this case the SNP increased their vote by 13%, Labour also increased their vote by 3% so the sum is (13-3)/2

which equals a five percent swing. Easy when you know how.