Additional Member System

The Additional Member System is an electoral system used to produce a more proportional result than Plurality voting, also known as First Past The Post. It is sometimes referred to as Mixed Member Proportional but these two electoral systems have different ways of allocating list seats. The term additional member system is almost only used in the United Kingdom but has been used to describe Italy's Electoral system from 1993 to 2005.

 Method of voting & calculation 

 Method of voting 
In the Additional Member System the voter has two votes ; one vote goes to an local representative or multiple representatives from a geographical electoral district. This is commonly referred as a constituency vote. The other vote goes to a political party. This is commonly referred as a list vote and is to represent the voter in a wider region.

 Calculation 
Firstly, the Constituency vote is counted: the votes are counted and the candidate with the most votes, even if the candidate did not reach a majority, wins the seat.

Next, each party's Member Score is calculated using a formula:

V = Party List Vote

S = Seat already won

Once the party's member score is calculated the list seats are given to each political party using the D'hondt formula.

 Example 
In this Hypothetical Example there is 100,000 voters and there are: 3 political Parties, 10 total Seats (6 constituency seats and 4 list seats).

PartyConstituency Party List Total 
SeatsTotal Seat
Share VotesVote
ShareSeatsVotesVote 
ShareSeatsParty X43,00043%445,00045%1550%Party Y36,00036%236,00036%1330%Party Z21,00021%019,00019%2220%

 List seats calculation 
PartyVotesMember
Score/1/2/3/4Seats
AllocatedParty X45,0009,000 9,0004,5003,0002,2501Party Y36,00012,00012,0006,0004,0003,0001Party z19,00019,00019,0009,5006,3334,7502

 Current usage 
 The London Assembly
 The Scottish Parliament
 The Welsh Assembly

 References 

Category:Voting systems