Mixed-member proportional representation

Mixed-member proportional representation is a hybrid two-tier voting system. MMP was originally used to elect representatives to the German Bundestag, which has also been adopted by Bolivia, Lesotho and New Zealand. It was used in Romania, in the 2008 and 2012 legislative elections. MMP is a hybrid method that uses party list proportional representation (PR) as its proportional component, and most commonly (but not necessarily) first-past-the-post voting (FPTP) as its district component. It is considered a mixed system (also known as a hybrid system or a semi-proportional representation system), which is a distinct voting system. "An electoral system is 'mixed' if more than one formula is employed to distribute legislative seats."Biproportional apportionment, first used in Zürich in 2006, is a hybrid method for adjusting an election's result to achieve overall proportionality.

In Germany, where it is used on the federal level and on most state levels, MMP is known as personalized proportional representation (personalisiertes Verhältniswahlrecht). In the United Kingdom such systems used in Scotland, Wales, and the London Assembly are referred to as Additional Member System. In the Canadian province of Quebec, where an MMP model was studied in 2007, it is called the compensatory mixed-member voting system (système mixte avec compensation or SMAC).

In most models the voter casts two votes: one for a constituency representative and one for a party. In the original variant used in Germany, both votes were combined into one, so that voting for a representative automatically meant also voting for the representative's party. Most of Germany changed to the two-vote variant to make local MPs more personally accountable. Voters can vote for the local person they prefer for local MP without regard for party affiliation, since the partisan make-up of the legislature is determined only by the party vote. In the 2005 New Zealand election, 20% of local MPs were elected from electorates (constituencies) which gave a different party a plurality of votes.

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