swing

A measure of the change between one election result and the next. As originally defined and used by D. E. Butler, ‘swing’ was the average of the winning party's gain in share of the vote and the losing party's loss. This formula, while a valuable summary measure that is still in daily use, suffers from two problems:

(1) It is hard to apply when more than two parties are in contention. Perhaps for this reason it has been little used for electoral analysis outside Britain, the United States, and Australasia.
(2) It averages percentages of one thing (the vote shares at the first election) with percentages of another (the vote shares at the second). So what is the resulting percentage figure a percentage of ?

 Ingenious but cumbersome ideas of triangular swing have been put forward to deal with the first problem (but how do you summarize four-party movement, for instance in Scotland?) The second problem leads statisticians to eschew ‘swing’ altogether. Matrix measures of electoral change could be substituted in the (rare) cases where details are available of every flow from one behaviour at the first election to another at the second. But no handy summary measure has been suggested.