Mixed-member proportional representation

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Mixed-member proportional representation (MMP or MMPR) is a type of representation provided by some mixed electoral systems which combine local winner-take-all elections with a compensatory tier with party lists, in a way that produces proportional representation overall. Like proportional representation, MMP is not a single system, but a principle and goal of several similar systems. Some systems designed to achieve proportionality are still called mixed-member proportional, even if they generally fall short of full proportionality. In this case, they provide semi-proportional representation.

In typical MMP systems, voters get two votes: one to decide the representative for their single-seat constituency, and one for a political party, but some countries use single vote variants. Seats in the legislature are filled first by the successful constituency candidates, and second, by party candidates based on the percentage of nationwide or region-wide votes that each party received.^[1][2][3] The constituency representatives are usually elected using first-past-the-post voting (FPTP). The nationwide or regional party representatives are, in most jurisdictions, drawn from published party lists, similar to party-list proportional representation. To gain a nationwide representative, parties may be required to achieve a minimum number of constituency seats, a minimum percentage of the nationwide party vote, or both.

MMP differs from mixed-member majoritarian representation (often achieved by parallel voting) in that the nationwide seats are allocated to political parties in a compensatory manner in order to achieve proportional election results across all seats (not just the additional seats). Under MMP, two parties that each receive 25% of the votes end up with about 25% of the seats, even if one party wins more constituency seats than the other. Depending on the exact system implemented in a country and the results of a particular election, the proportionality of an election may vary.^[4] Overhang seats may reduce the proportionality of the system, although this can be compensated for by allocating additional party list seats to cover any proportionality gap.^[5]

The specific system of New Zealand for electing its parliament is called MMP, while in other countries similar systems are known under other names.