Swing (United Kingdom)
Swing, in British politics, is a number used as an indication of the scale of voter change between two political parties. It originated as a mathematical calculation for comparing the results of two constituencies. Britain uses a first-past-the-post voting system. The swing (in percentage points) is the percentage of voter support minus the comparative percentage of voter support corresponding to the same electorate or demographic.
The swing is calculated by comparing the percentage of voter support from one election to another. The percentage value of the comparative elections results are compared with the corresponding results of the substantive election. An electoral swing analysis shows the extent of change in voter support from one election to another. It can be used as a means of comparison between individual candidates or political parties for a given electoral region or demographic.
The original mathematical construct Butler swing is defined as the average of the Conservative percentage-point gain and Labour percentage-point loss between two elections, calculated on the basis of the total number of votes (including those cast for candidates other than Conservative or Labour). There is an alternative version called Steed swing which calculates the swing on the basis of votes cast for Conservative and Labour only. It is possible for the same election to have a Butler swing of one sign and a Steed swing of the other.
As an example, assume that a constituency had two sequential election results as follows:
The Butler swing to the Conservatives is therefore:
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