|Part of the Politics series|
A Mixed-Member System, also known as a hybrid voting system, or Additional Member System, is a multi-winner voting system which combines the principles of proportional representation and plurality voting.     The most common mixed electoral systems are: Mixed Member Proportional, Alternative Vote Plus, Additional Member System, and majority bonus system. Mixed-Member Systems are typically semi-proportional, but some forms of MMP can be highly proportional.
Mixed-Member Systems in the broader family of voting systems
Most experts group electoral systems into 3 general categories:
- Proportional Representation systems;
- Mixed-Member systems; and
- Single-winner systems;  :22
|Proportional Representation Systems||Mixed Member Systems||Single-winner Systems|
|Single Transferable Vote||Mixed Member Proportional||First Past the Post|
|Party List Proportional Representation (closed/open/local)||Alternative Vote Plus||Alternative Vote/Instant-runoff voting|
|Additional Member System||Preferential block voting|
|Majority Bonus System||Limited Vote|
While most proportional representation systems are to some extent semi proportional due to thresholds, or split electoral regions, this article deals more with mixed systems, that are to an extent proportional but not designed to be as proportional as possible across an entire country.
The choice to use a semi-proportional voting system may be a deliberate attempt to find a balance between majority rule and proportional representation: semi-proportional systems can allow for fairer representation of those parties that have difficulty gaining individual seats while still keeping the possibility of one party gaining a majority when there is a landslide victory.
Because there are many measures of proportionality, and because there is no objective threshold, opinions on what constitutes a semi-proportional method rather than a majoritarian or a fully proportional one, may differ.
Methods where parties can only achieve proportionality by coordinating their voters are usually considered to be semi-proportional. They are not majoritarian, since in the perfect case, the outcome will be proportional, but they are not proportional either, since such a perfect case requires a very high degree of coordination. Such methods include the single non-transferable vote and limited voting, the latter of which becomes less proportional the more votes each voter has. The cumulative voting also allows minority representation, concentrating votes over the number of candidates that every minor party thinks it can support.
This group of system are, at least technically, non-partisan. Surely, group of candidates can coordinate their campaigns, and politically present themselves as party members, but there is no obligation for electors to respect these party links, and forms of panachage are usually possible.
Single Transferable Vote
Some consider STV to be a semi proportional system. The degree of proportionality across the a country depends on the average size of constituency used, in the 2011 Irish general election Fine Gael came just 9 (4.8%) seats short of an overall majority with just 36.1% of the vote, however the result of the election was exceptional and Fine Gael benefited from a high level of transfers from those who did not rank them first. It is possible under STV to win an overall majority with significantly fewer than 50% of the votes, but this is only if the party also gains a high level of transfers from those who do not rank them first. As it lacks any arbitrary nationwide election threshold even with the Irish 3-5 seat system the level of proportionality does not veer too far from countries with such thresholds.
Mixed Member Proportional
Mixed member proportional representation (MMP), also called the additional member system (AMS), is a hybrid, two-tier, system combining a non-proportional single-winner election and a compensatory regional or national party list PR one. MMP is similar to forms of proportional representation (PR) in that the overall total of party members in the elected body is intended to mirror the overall proportion of votes received; it differs by including a set of members elected by geographic constituency who are deducted from the party totals so as to maintain overall proportionality. Other forms of semi-proportional representation are based, or at least use, party lists to work. Looking to the electoral systems effectively in use around the world, there are three general methods to reinforce the majority rule starting from basic PR mechanisms: parallel voting, majority bonuses, and extremely reduced constituency magnitude. An additional member system may reinforce majorities if the proportion of compensatory seats is too low.
Alternative Vote Plus
The Alternative Vote Plus (AV+), or Alternative Vote Top-up, is a mixed voting system. AV+ was invented by the 1998 Jenkins Commission which first proposed the idea as a system that could be used for elections to the Parliament of the United Kingdom.
As the name suggests, AV+ is an additional member system which works in two parts: the 'AV' part and the 'plus' part. As in the Alternative Vote Instant-runoff voting system, candidates are ranked numerically in order of preference. The important difference is that an additional group of members would be elected through the regional party lists system to ensure proportionality; in typical proposals, these members are 15–20% of the whole body. More specifically, each voter would get a second vote to elect a county or regional-level representative from a list of candidates of more than one person per party. The number of votes cast in this vote would decide how many representatives from that county or region would go on to parliament.
Additional Member System
The additional member systems where the additional members are not sufficient to balance the disproportionality of the original system can produce less than proportional results, especially in the National Assembly for Wales where only 33.3% of members are compensatory. The electoral system commonly referred to in Britain as the "additional member system" is also used for the Scottish Parliament, and the London Assembly, with generally proportional results.
Majority Bonus System
A majority bonus system introduce a FPTP-like idea in multi-member constituencies. The bonus gives additional seats to the first party or alliance, to create a landslide victory as it happens in countries using the FPTP even if single-member constituencies are not in use. The majority bonus system was firstly introduced by Benito Mussolini to win the election of 1924, but it was later used in Italy again, with additional democratic limits, and then expanded in some neighbor countries like San Marino, Greece and France.
The most simple mechanism to reinforce major parties in PR system is a severely reduced constituency magnitude, so to reduce the possibility for minor national parties to gain seats. If the Spanish electoral system is still considered a form of proportional representation, the binomial system used in Chile effectively establish by law a two-party rule over the country.
The last main group usually considered semi-proportional consists of parallel voting models. The system used for the Chamber of Deputies of Mexico since 1996 is considered a parallel voting system, modified by a list-seat ceiling (8%) for over-representation of parties. The "scorporo" system used for the Parliament of Italy from 1993 to 2005 and the electoral system for the National Assembly of Hungary since 1990 are also special cases.
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