Mixed-Member Systems

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.

Most experts group electoral systems into 3 general categories:

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.

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