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Electoral swing is often used to analyse the performance of parties in different areas. Election swings show the extent of change in voter support for a political party, from one election to the next, and is typically expressed as a positive or negative percentage point change.

This paper shows how to calculate two-party swing using the ‘Butler’ method: swing is defined as the average of the percentage point gain of party A and the percentage point loss of party B. 

This method of calculating swing works well in situations where two parties compete in elections over time, but is of more limited use when several parties compete for the same seat. 

National swing at general elections

The chart below shows swings between the two major parties (Conservative and Labour) at general elections since 1945. A negative swing indicates a swing from the Conservatives to Labour.

Swing at national elections

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