Voting swings and seat changes: A stochastic model

The usual method for analysing the relationship between net seat changes and net voting swings has been Mackerras's pendulum, which is based on the assumption of uniform swing. This paper argues that the probabilistic concepts which underlie the relationship have not been clearly recognised in the past, and proposes a simple stochastic model for net swings. The pendulum‐uniform swing model is a special case of this model. A mathematical analysis is given of the implications of the model for the relationship between swing and seat changes. As well as showing how net seat changes can be predicted from net voting swing, the analysis demonstrates that quite large fluctuations in net seat changes can result from random variation alone.