Driven by data; ridden with liberty.
The notion of a swing in an election is commonly-used by broadcasters, newspapers and the general public.
Louise Mensch, the former Conservative MP for Corby, suggested that The Independent had misreported a swing from Remain to Leave. This 10-point swing was in an Ipsos MORI poll for the EU referendum, which is being held on Thursday 23rd June.
Mrs Mensch wrote on Twitter:
The independent reports as a ten point swing to [Brexit] what I think is a 20 point swing to [Vote Leave]
Douglas Carswell MP (UKIP, Clacton) replied to Mrs Mensch:
sssshhhhh! Don’t tell them. They can’t count
This is, inarguably, incorrect. Electoral swing is defined as the average movement from one party to another. The House of Commons Library states :
Electoral swing is a way of comparing the results of two elections in the same area, whether nationally or locally. It is often used to analyse the performance of parties over time or in one election between different electoral areas. The basis of calculating swing is each party’s percentage share of the vote.
The swing from Party A to Party B is conventionally defined as the average of the percentage point fall in Party A’s share of the vote and the percentage point rise in Party B’s.
A swing of 5 per cent from Party A to Party B can be visualised as 5 percentages points’ worth of voters who previously voted for Party A voting instead for Party B. From a position of parity at the previous election, a swing of 5 percentages points would leave Party B with a 10 percentage point majority over Party A.
The conventional calculation of swing incorporates all the votes cast in an area, not just those for the parties in question.
To be more precise, this calculated swing is called the Butler swing. There is another type of swing called the Steed swing, which considers only the votes cast for the two parties in each election.
To work through a selected example, the constituency of Corby had a Conservative candidate called Louise Bagshawe in 2010 .
The Conservative vote share rose by 2.3 percentage points, and Labour’s vote share fell by 4.5 percentage points. The swing from Labour to the Conservatives is the average of Labour’s fall and the Conservative rise: 3.4 percentage points.
That was Mrs Mensch’s swing in 2010, and matches the BBC report on the constituency.
 Clements, R., 2010. Electoral swing. Available from: http://researchbriefings.parliament.uk/ResearchBriefing/Summary/SN02608 [Accessed: 18th June 2016]
 BBC, 2010. Election 2010: Corby. Available from: http://news.bbc.co.uk/1/shared/election2010/results/constituency/b20.stm [Accessed: 18th June 2016]