Additional Member System (AMS)

“The Additional Member System (AMS) is the term used in the United Kingdom for the voting system used in Scotland, Wales, and the London Assembly.

“AMS is sometimes confused with the mixed member proportional representation system… some representatives are elected from single-winner geographic constituencies and others are elected under proportional representation from a wider area, usually by party lists.  Voters usually have two votes, one for the party and the second for the candidate in a constituency…

“Under [MMPR] … 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 seats to offset all or most of the disproportionate result in the constituencies.  The party vote determines the proportional number of representatives the party has in the assembly, so creating a proportional representation…

“The Additional Member System is almost the same, except that there are no ‘overhang’ or ‘balance’ seats to compensate if a party [gets] more constituency seats than its party vote should allow, and so it is, in some cases, not proportional. …” — Additional Member System

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