Additional member system

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The Additional Member System (AMS) is a branch of voting systems in which some representatives are elected from geographic constituencies and others are elected under proportional representation from party lists. The constituency representatives are generally elected under the first-past-the-post voting system. The party list representatives are elected by a second vote, where the electors vote for a political party, not directly for an individual. The particular individuals selected come from lists drawn up by the political parties before the election, at a national or regional level.

Variations of the AMS have different ways of determining how many party list representatives each party is entitled to. The main difference between systems is whether the constituency representatives are counted when list representatives are allocated to each party.

Under the Mixed Member Proportional (MMP) or Top-Up system, the aim is either for the party's total number of representatives, including constituency representatives, to be proportional to its percentage of the party vote, or for the allocation of additional party list seats to offset some or all of the disproportionate result in the constituencies. The party-list vote largely determines the number of representatives the party has in the assembly.
Under the Parallel Voting or Supplementary Member (SUP) system, the party list seats are allocated proportionally within themselves, so any constituency seats the party may have won are additional.

Parallel Voting is the more common variation among voting systems of the world. Small parties will generally win more seats under MMP than SUP unless there is a threshold of exclusion, such as the 5% or 3 constituencies threshold in Germany, or the 5% or 1 constituency seat threshold in New Zealand.

Additional member system

See also