Mackerras pendulum

The Mackerras pendulum was devised by the Australian psephologist Malcolm Mackerras as a way of predicting the outcome of an election contested between two major parties in a Westminster style lower house legislature such as the Australian House of Representatives, which is composed of single-member electorates and which uses a preferential voting system such as a Condorcet method or IRV.

Given a uniform swing to the opposition or government parties, the number of seats that change hands can be predicted.

The two-party preferred (2PP) method of prediction attempts to estimate the flow of second and subsequent preferences from smaller parties in order of their expected elimination during the instant-runoff voting process, to establish ultimately which major party the voters will choose – Labor or Coalition (Liberal/National) in the Australian context.

Whichever party polls the higher two-party-preferred percentage at the election usually holds the majority of seats to form government.

Mackerras has taken account of fully distributed transferable preference votes since the 1983 federal election.