A mixed electoral system or mixed-member electoral system combines methods of majoritarian and proportional representation (PR). The majoritarian component is usually first-past-the-post voting (FPTP/SMP), 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, 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 (most often single-member constituencies) with regional or national (multi-member constituencies) representation, having multiple tiers. 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, 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 . Most mixed systems have all the voters contributing to the election of both groups of members.
Types of mixed systems
Compensatory/non-compensatory seat allocation
A major distinction is often made between mixed compensatory systems and mixed non-compensatory systems. 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, 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 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.
In non compensatory system, each party wins its proportional share of the 100 PR seats. Under such a system, the total number of seats (FPTP + PR) received by each party would not be proportional to its share of the popular vote. Party A receives just slightly less of the popular vote than Party B, but receives significantly more seats. In addition to its success in the district contests, Party A receives almost as many of the PR seats as Party B.
If the PR seats are allocated in a compensatory system, the total number of seats awarded to each party is proportional to the party's share of the popular vote. Party B wins 33 of the district seats and its proportional share of the 200 seats being filled is 80 seats (40 percent of the total 200 seats) (the same as its share of the popular vote) so it is awarded 47 of the PR seats.
In practice, compensatory seat allocation is complicated by the possibility that one or more parties wins so many of the district seats ("overhang") that the available number of PR seats is insufficient to produce a fully proportional outcome. Some mixed compensatory systems have rules that address these situations by adding additional PR seats to achieve overall PR. These seats are used only until the next election, unless needed again at that time.