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The Additional Member System (AMS) is the term used in the United Kingdom for the mixed member proportional representation voting system used in Scotland, Wales and the London Assembly.
Also, AMS as a generic term could refer to either the mixed member proportional representation system or the parallel voting system, a semi-proportional voting system. In both cases, 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, even if sometimes these votes are combined. The constituency representatives are generally elected under the first-past-the-post voting system. The party representatives are elected by a party vote, where the electors vote for a political party, and usually not directly for an individual. The particular individuals selected come from lists drawn up by the political parties before the election, at a national or regional level. Basic technical and political differences intervene between these two systems.
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At the regional or national level (i.e. above the constituency level) several different calculation methods have been used:
As in many systems containing or based upon party-list representation, in order to be eligible for list seats in some AMS models, a party must earn at least a certain percentage of the total party vote, or no candidates will be elected from the party list. Candidates having won a constituency will still have won their seat. In almost all elections in the UK there are no thresholds except the "effective threshold" inherent in the regional structure. However the elections for the London Assembly have a threshold of 5% which has at times denied seats to the Christian People's Alliance (in the 2000 election), the British National Party and Respect – The Unity Coalition (both in the 2004 election). Under the AMS used in Italy from 1993 to 2005, a threshold of 4% was needed to receive proportional seats.
So-called "decoy lists" are a trick to unhinge the compensation mechanisms contained into the proportional part of the AMS, so to de facto establish a parallel voting.
For instance in the Italian general election, 2001, one of the two main coalitions (the House of Freedoms, which opposed the MMP system), linked many of their constituency candidates to a decoy list (liste civetta) in the proportional parts, under the name Abolizione Scorporo. As a defensive move, the other coalition, Olive Tree, felt obliged to do the same, under the name Paese Nuovo. The constituency seats won by each coalition would not reduce the number of proportional seats they received. Between them, the two decoy lists won 360 of the 475 constituency seats, more than half of the total of 630 seats available, despite winning a combined total of less than 0.2% of the national proportional part of the vote. In the case of Forza Italia (part of the House of Freedoms), the tactic was so successful that it did not have enough candidates in the proportional part to receive as many seats as it in fact won, missing out on 12 seats.
Decoy lists are not used in Scotland, Wales, or most other places using AMS, where most voters vote for candidates from parties with long-standing names. In the run up to the 2007 Scottish election, the Labour party had considered not fielding list candidates in the Glasgow, West of Scotland, and Central Scotland regions, as their constituency strength in the previous two elections had resulted in no list MSPs; instead they proposed to support a list composed of Co-Operative Party candidates; previously the Co-Operative party had chosen not to field candidates of its own but merely to endorse particular Labour candidates. However the Scottish Electoral Commission ruled that as membership of the Co-Op party is dependent on membership of the Labour party they could not be considered distinct legal entites. In contrast, in the 2007 Welsh Assembly Election, Forward Wales had its candidates (including sitting leader John Marek) stand as independents, to attempt to gain list seats they would not be entitled to if Forward Wales candidates were elected to constituencies in the given region. However the ruse failed: Marek lost his seat in Wrexham and Forward Wales failed to qualify for any top-up seats.
Decoy lists would be technically useless in most mixed-member proportional voting models: there, because the proportional party vote is the sole to determine the final political result of the election, overhang seats would automatically compensate any effect of eventual decoy lists.
In 1976, the Hansard Society recommended that AMS in a form different from the German be used for UK parliamentary elections, but instead of using closed party lists, it proposed that seats be filled by the 'best runner-up' basis used by the German state of Baden-Wurttemberg, where the compensatory seats are filled by the party's defeated candidates who were the 'best near-winner' in each of the state's four regions. The system eventually adopted without that provision for elections to the Scottish Parliament, Welsh Assembly and London Assembly, but not for elections to the House of Commons.
A similar system was proposed by the Independent Commission in 1999, known as Alternative vote top-up (AV+). This would have involved the use of the Alternative Vote for electing members from single-member constituencies, and regional open party lists. However, contrary to the Labour Party's earlier manifesto promises, no referendum was held before the 2001 general election and the statement was not repeated.
The AMS system in use in the London Assembly would have been used for the other proposed regional assemblies of England, but this process has stalled since the No vote in the Northern England referendums in 2004.
Since smaller parties are likely, in compensatory systems, to win a larger number of proportional seats, such additional member systems hand additional political power to the leaders of these parties at the expense of regional directly elected representatives, unless the additional members are elected on an open regional list or a closed regional list as in Scotland and Wales. With closed lists, party-list candidates may become puppets for the party leadership, or may add diversity to the party's elected members. The largest party in an election is likely to win a smaller number of proportional seats, so that governing parties may lose diversity, unless the members elected from the party list when it was in opposition then win local seats when the party gains enough support to form the government.
In the parallel systems, even the largest party will elect members from the party list, so the top list positions are guaranteed seats. This system is found in emerging democracies like post-communist Russia, where new national parties were evolving, and the voting system was intended to foster them, while allowing local independent members to win local seats, many of whom then joined the winning party. It retains the plurality principle but has another paper to allow voting for a party rather than a candidate.
Some writers argue that AMS is difficult for voters to understand, and thus disenfranchises the very people it is meant to empower. There is some evidence that many Scottish voters did not understand the implications of the system at first. In the first election for Scotland's new Parliament, the majority of voters surveyed misunderstood some key aspects of the difference there between the "first" (constituency) vote and the "second" (regional list) vote; indeed in some ways the understanding worsened in the second election. The Arbuthnott Commission found references to first and second votes fuelled a misperception that the constituency vote should be a first preference and the regional vote a second one. That misperception was not helped by the Green Party's tactic of running only regional candidates and appealing for "second votes."
In Scotland, to deal with the misunderstanding between "first" and "second" votes, the ballot for the 2007 Scottish Parliament election was changed as recommended by the Arbuthnott Commission. The British government announced on 22 November 2006 that the two separate ballot papers used in the previous Scottish Parliament elections would be replaced for the elections in May 2007 by a single paper — with the left side listing the parties standing for election as regional MSPs and the right side the candidates standing as constituency MSPs.
| Scottish Parliament Election Study 1999 and Scottish Social Attitudes Survey 2003 | % answering correctly | |
|---|---|---|
| Question (and correct response) | 1999 | 2003 |
| You are allowed to vote for the same party on the first and second vote (True) | 78% | 64% |
| People are given two votes so that they can show their first and second preferences (False) | 63% | 48% |
| No candidate who stands in a constituency contest can be elected as a regional party list member (False) | 43% | 33% |
| Regional party list seats are allocated to try to make sure each party has as fair a share of seats as is possible (True) | 31% | 24% ? |
| The number of seats won by each party is decided by the number of first votes they get (False) | 30% | 42% |
| Unless a party wins at least 5% of the second vote, it is unlikely to win any regional party lists seats (True) | 26% | 25% |
| Average | 45% | 39% |
However, the detailed results of the Scottish Social Attitudes Survey, 2003 (shown in the table above) show the confusion was about "first" and "second" votes, creating an average of 28% wrong answers.