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.
The pendulum works by lining up all of the seats held in Parliament for the government, the opposition and the cross benches according to the percentage point margin they are held by on a two party preferred basis. This is also known as the swing required for the seat to change hands. 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. A Coalition 2PP of 51% would mean a Labor 2PP of 49% and vice versa. Whichever party polls the higher two-party-preferred percentage at the election usually holds the majority of seats to form government. Exceptions to this since 2PP was introduced in 1949 were in 1954 (49.3%), 1961 (49.5%), 1969 (49.8%), 1990 (49.90%), and 1998 (49.02%). 1940 was estimated to have been won on 49.7%.
Mackerras has taken account of fully distributed transferrable preference votes since the 1983 federal election. Previously, he estimated a two-party-preferred outcome from limited, selective consideration of preferences.
The largest two-party-preferred election result for the Liberal Party of Australia was at the 1966 federal election, on 56.9%, while the largest two-party-preferred election result for the Australian Labor Party was at the 1983 federal election, on 53.23 percent. The largest unofficial result was 58.2% for Labor at the 1943 federal election, estimated by Mackerras.
Considering two-party-preferred estimates going back to the 1949 election, the swing to Labor at the 2007 federal election was the third-largest two-party swing, behind Malcolm Fraser and the coalition in 1975 on 7.4% and Gough Whitlam and Labor in 1969 on 7.1%.
- Mackerras federal election pendulum, 2006
- Post-election pendulum for the 2007 Australian federal election
- Post-election pendulum for the Australian federal election, 2010
- Pre-election pendulum for the next Australian federal election
- Swingometer, a graphical device with similar functionality used in the United Kingdom since 1955
- Goot M Three strikes against the polls, or the Govt is out) Australian Broadcasting Corporation, Opinion, 1 October 2007