Mixed electoral system
A mixed electoral system or mixed-member electoral system combines methods of majoritarian and proportional representation (PR).[1][2][3] The majoritarian component is usually first-past-the-post voting (FPTP/SMP),[4] whereas the proportional component is most often based on party-list PR. The results of the combination may be mixed-member proportional (MMP), where the overall results of the elections are proportional,[2] or mixed-member majoritarian, in which case the overall results are semi-proportional, retaining disproportionalities from the majoritarian component.
Mixed-member systems also often combine local representation[5] (most often single-member constituencies) with regional or national (multi-member constituencies) representation, having multiple tiers.[6] This also means voters often elect different types of representatives who might have different types constituencies. Some representatives may be elected by personal elections where voters vote for candidates, and some by list elections where voter vote primarily for electoral lists of parties.
In most mixed systems, every voter can influence both the district-based and PR aspects of an election,[7] such as under parallel voting; in others, the voter casts just one vote (mixed single vote), which is used to contribute to both a personal (usually district) election and to the filling of seats through list system.[8] Most mixed systems have all the voters contributing to the election of both groups of members.
Types of mixed systems[edit]
Compensatory/non-compensatory seat allocation[edit]
A major distinction is often made between mixed compensatory systems and mixed non-compensatory systems.[8] In both types of systems, one set of seats is allocated using a plurality or majoritarian method, usually First past the post. The remaining seats are allocated to political parties partially or wholly based on a proportional allocation method such as highest averages or largest remainder. The difference is whether or not the results of the district elections are considered when allocating the PR seats.
In mixed non-compensatory systems, such as parallel voting,[4] the proportional allocation is performed independently of the district election component.
In mixed compensatory systems, the allocation of the top-up seats is done in such a way as to compensate as much as possible for dis-proportionality produced by the district elections. MMP generally produces proportional election outcomes, meaning that a political party that wins n percent of the vote will receive roughly n percent of the seats.
The following hypothetical example based on the one by Massicotte[4] illustrates how "top-up" PR seats are typically allocated in a compensatory system and in a non-compensatory system. The example assumes a 200-seat legislative assembly where 100 seats are filled using FPTP and the other 100 seats are awarded to parties using a form of PR. The table below gives the popular vote and FPTP results. The number of PR seats allocated to each party depends on whether the system is compensatory or non-compensatory.