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:

${\displaystyle {V \over S+1}}$

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).

Party	Constituency			Party List			Total Seats	Total Seat Share
	Votes	Vote Share	Seats	Votes	Vote Share	Seats
Party X	43,000	43%	4	45,000	45%	1	5	50%
Party Y	36,000	36%	2	36,000	36%	1	3	30%
Party Z	21,000	21%	0	19,000	19%	2	2	20%

List seats calculation

Party	Votes	Member Score	/1	/2	/3	/4	Seats Allocated
Party X	45,000	9,000	9,000	4,500	3,000	2,250	1
Party Y	36,000	12,000	12,000	6,000	4,000	3,000	1
Party z	19,000	19,000	19,000	9,500	6,333	4,750	2

Current usage

• The London Assembly^[1]
• The Scottish Parliament^[2]
• The Welsh Assembly^[3]