Mackerras Pendulum

From Wikipedia, the free encyclopedia

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.

[edit] Two Party Preferred

The candidate with the least amount of votes is wiped out and redistributed to the voter's next choice until two candidates are left to gain a two party preferred figure.

Two party preferred polling by Newspoll, ACNielsen, Roy Morgan and Galaxy between the 2004 and 2007 federal election.

The two party preferred (2PP) system works by distributing preferences of smaller parties, wiped out in the instant-runoff voting process, to establish ultimately which major party the voter will choose - Labor or Coalition (Liberal or National). A coalition 2PP of 51 percent would mean a Labor 2PP of 49 percent, or vice versa. Whichever party polls the higher two-party preferred figure 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 be won on 49.7 percent.

Votes have only been fully distributed since the 1983 federal election, previous to this, limited distributions occurred to achieve an estimated two-party preferred result.

The largest two-party preferred election result for the Liberal Party of Australia was at the 1966 federal election on 56.9 percent, 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 percent for Labor at the 1943 federal election, estimated by Malcolm Mackerras [1]).

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 per cent, and Gough Whitlam and Labor in 1969 on 7.1 per cent.