Additional member system

The Additional Member System (AMS), also known as mixed-member proportional representation (MMP) outside of the United Kingdom is a hybrid two-tier mixed-member system which varies in proportionality. It also belongs to the larger group of Mixed-Member Systems.

The term AMS is used in Scotland, Wales, and the London Assembly.

In an election using the Additional Member System, each voter casts two votes: a vote for a candidate standing in their constituency (with or without an affiliated party), and a vote for a party list standing in a wider region made up of multiple constituencies. The constituency vote is used to elect a single representative in the voter's constituency using the traditional First-Past-The-Post system: the candidate with the most votes (not necessarily a majority of the electorate) wins. The regional vote is used to elect representatives from party lists to stand in regional seats, taking into account how many seats were gained by that party in the constituency vote, using a system of proportional representation: the number of seats a party receives will roughly reflect its percentage of the vote.

Voters are not required to vote for the same party in the constituency and regional votes. If a voter votes for different parties at the constituency and regional levels this is referred to as split-ticket voting. In the regional vote, the voter votes for a specific party, but has no control over which candidates from the party are elected. On the other hand, in the constituency vote, the voter votes for a specific candidate rather than a party. The Arbuthnott Commission recommended that Scotland change to a model where the voter can vote for a specific regional candidate as well, but this has not been implemented. With a similar model in Bavaria the second vote is not simply for the party but for one of the candidates on the party's regional list and both votes count for party and candidates so that every vote counts: Bavaria uses seven regions for this purpose. In Baden-Württemberg there are no lists; they use the "best near-winner" method in a four-region model, where the regional members are the local candidates of the under-represented party in that region who received the most votes in their local constituency without being elected in it, but this model has not been copied in the United Kingdom.

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