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Concept
Additional-member system
Summary
The additional-member system (AMS) is a mixed electoral system under which most representatives are elected in single-member districts (SMDs), and the other "additional members" are elected to make the seat distribution in the chamber more proportional to the way votes are cast for party lists. It is distinct from parallel voting (also known as the supplementary member system) in that the "additional member" seats are awarded to parties taking into account seats won in SMDs (referred to as compensation or "top-up"), which is not done under parallel voting (a non-compensatory method).
AMS is categorised under semi-proportional electoral systems, differently from the mixed-member proportional representation (MMP). In practice, the way that proportional additional member systems work depends on the number of additional ("top-up") seats and the votes cast in a specific election.
This article focuses primarily on semi-proportional implementations of AMS, like the ones used in the UK. AMS is used by the Scottish Parliament, the Welsh Parliament and the London Assembly. In Scotland and Wales list members ("top-up" seats) are elected by region; in London there is a single London-wide pooling of list votes.
In an election using the additional member system, each voter usually casts two votes: a vote for a candidate standing in their local constituency (with or without an affiliated party), and a vote for a party list standing in a wider region made up of multiple constituencies (or a single nationwide constituency).
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 main variation was the 1993 Italian electoral law for the Senate, later abolished in 2005.
Official source
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