Additional Member System

The Additional Member System (aka Mixed Member Proportional Representation) is a voting system incorporating an element of proportional representation. It is a combination of the First-past-the-post election system and closed list party-list proportional representation, used for multiple-winner elections. In this system, the district (e.g. the country, region, or state) is carved into constituencies, each with one representative. In addition to these constituency representatives, the assembly also has representatives not tied to a particular constituency, but elected from regional party lists.

This fictional New Zealand ballot has the party vote on the left and the constituency vote on the right. (Click here for larger image)
Photo credit: Crown copyright

Table of contents

1 Employment
2 Procedures
3 Overhang Seats
4 Threshold

Employment

The AMS is used to elect members to numerous representative bodies around the world.

Germany
- Bundestag (Federal lower house)
Japan
- House of Representatives (lower house of Kokkai Diet)
New Zealand
- House of Representatives (unicameral Parliament)
United Kingdom
- Scottish Parliament
- Welsh Assembly
- the Independent Commission on the Voting System headed up by Lord Jenkins proposed a variant of AMS called AV+ in which constituency seats would be filled using Instant Runoff and an open-list would be used. The governing Labour party made a manifesto commitment to put the system to the public in a referendum.

Procedures

The voter makes two votes: one for a constituency representative and one for a party.

In each constituency, the representative is chosen via first-past-the-post (i.e. the representative with the most votes wins).

On the district level, the total number of seats in the assembly are allocated to parties proportionally to the number of votes the party received in the party portion of the ballot. Subtracted from each party's allocation is the number of constituency seats that party won. The number of seats remaining allocated to that party are filled using the party's list. If a candidate is on the party list, but wins a constituency seat, they do not receive two seats; they are instead crossed off the party list and replaced with the next candidate down.

Overhang Seats

Because a party can gain less seats by the party vote than needed to justify the won constituency seats, overhang seats can occur. There are different ways of dealing with overhang seats. In the Scottish Parliament the number of overhang seats is taken from the number of proportional seats of the other parties, in Germany's Bundestag the overhang seats remain and in New Zealand the other parties get compensatory seats to obtain the proportionality.

Threshold

In order to be eligible for list seats in the New Zealand and German systems, a party must either earn at least 5% of the party vote or must win at least one constituency seat (three constituency seats in Germany). This is extremely important to the minor parties. A party which wins no constituency seats and fails to meet the 5% threshold faces oblivion. Having a leader with a safe constituency seat is a tremendous asset to a minor party in such a system as it ensures survival.