The phrase Additional Member System is often used to describe mixed member proportional voting where the results in the non-proportional election are taken into account and (either wholly or partly) compensated in the proportional system; this is the system described here. If each of the two election counts do not have a substantial impact on the result of the other, then parallel voting is the typical description, and is in fact a more common system worldwide.

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

Table of contents

1 Employment 1.1 Proposal for British Elections 2 Procedures 2.2 Overhang Seats 2.3 Threshold 3 Potential for Tactical Voting

Employment

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

• Germany
  • Bundestag (Federal lower house)
• New Zealand
  • House of Representatives (unicameral Parliament)
• United Kingdom
  • Scottish Parliament
  • Welsh Assembly

Proposal for British Elections

The British Labour Party stated in its 1997 manifesto that it would put the question of a proportional electoral system to the public in a referendum. 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. There was not a referendum before the 2001 election and the statement was not repeated.

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 or national level (i.e. above the constituency 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.

Potential for Tactical Voting

In terms of tactical voting, the first vote for the district representative is often much less important than the second party list vote in determining the overall result of an election; in other cases a party may be so certain of winning seats in the district election that it expects no extra seats in the proportional top-up. Some voters may therefore seek to get a double representation by voting tactically and splitting their votes, though this runs the risk of unintended consequences.