Most proportional representation systems do not yield precisely proportional outcomes due to the use of election thresholds, small electoral regions, or other implementation details that vary from one elected body to another. This article deals primarily with systems inherently designed to produce moderately proportional election results.
The choice to use a semi-proportional electoral system may be a deliberate attempt to find a balance between single-party rule and proportional representation. Semi-proportional systems can allow for fairer representation of those parties that have difficulty gaining even a single seat while retaining the possibility of one party gaining an overall majority of seats even if it receives less than a majority of the votes; they can ensure that the two or three largest parties all have their due share of seats or more while not producing representation for the smallest parties.
Because there are many measures of proportionality,[4][5] and because there is no objective threshold, opinions may differ on what constitutes a semi-proportional system as opposed to a non-proportional one or a fully proportional system.
Non-partisan systems
Election systems in which a party can achieve its due share of seats (proportionality) only by coordinating itsr voters are usually considered to be semi-proportional.[6] They are not non-proportional or 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 systems include the single non-transferable vote and limited voting, the latter of which becomes less proportional the more votes each voter has. (under limited voting, coordination of voters is often secured by limiting the size of the party slate to the number of votes each voter has.) The cumulative voting also allows minority representation, but only by concentrating votes on the number of candidates that a minor party thinks it can elect, often just one.
This group of non-partisan systems is, at least technically, non-partisan. Certainly, a group of candidates can coordinate their campaigns, and politically present themselves as party members, but there is no obligation for electors to respect those party links, and forms of panachage are usually possible.
Some consider STV to be a semi-proportional system.[7] Others describe it as a proportional system.[8] The degree of proportionality of the results in a district (and when combined with other district results, the proportionality of results across a country) depends on the number of seats elected in the district. In the 2011 Irish general election, Fine Gael came nine seats (4.8%) short of an overall majority with just 36.1% of the first preference votes. Ireland uses a range of sizes of districts, three to five. The election results were exceptional -- Fine Gael benefited from transfers from those who did not rank them first, from voters whose back-up preferences allowed their vote to cross party lines.
Under STV a party can win an overall majority with significantly fewer than 50% of the first preference votes. This usually happens when a party gains many vote transfers from those who did not give their first preference to that party. Ireland lacks any arbitrary nationwide election threshold, which can also produce dis-proportional results. In other countries where an electoral threshold is used, some parties are arbitrarily blocked off from taking seats. Thus even with Ireland's somewhat small 3 to 5 seat District Magnitude, the level of proportionality in Ireland does not veer too far from PR countries with electoral thresholds.
Partisan systems
Other forms of semi-proportional representation are based on, 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 majoritarian principle of representation (but not necessarily majoritarianism or majority rule, see electoral inversion and plurality) starting from basic PR mechanisms: parallel voting, the majority bonus system (MBS), and extremely reduced constituency magnitude.
In additional member systems (AMS), the number of additional members may not be sufficient to balance the disproportionality of the original system, thereby producing less than proportional results. When this imbalance is created intentionally, the result could be described as a semi-proportional system — for example, 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. Similarly, in vote transfer based mixed single vote systems, the number of compensatory seats may be too low (or too high) to achieve proportionality,[9] such a system is used in Hungary in local elections[10] 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, based on parallel voting, but also including compensatory mechanisms – which however are insufficient for providing proportional results.
A majority bonus system takes an otherwise proportional system based on multi-member constituencies, and introduces disproportionality by granting additional seats to the first party or alliance. Majority bonuses help produce landslide victories similar to those which occur in elections under plurality systems. The majority bonus system was first introduced by Benito Mussolini to win the election of 1924, then it was later used in Italy again, with additional democratic limits, and then again expanded in some neighboring countries like San Marino, Greece and France.
The simplest 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 Spanishelectoral system is still considered a form of proportional representation, the binomial voting system used in Chile effectively establishes 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.
80% of seats (rounded to the nearest integer) in each constituency are awarded to the party receiving the most votes (party block voting), remaining seats are allocated proportionally to other parties receiving over 10% (closed list, D'Hondt method)
Local electoral districts within country/capital borders and a single nationwide constituency that includes non-resident with Hungarian citizenship as well
Two-round system (TRS) in single-member districts, two-round block voting (BV) in dual-member districts, and List PR (simple quota largest remainder; closed-list) in larger districts + twice 20 nationally List PR (one set of 20 reserved for women)
First-past-the-post (FPTP/SMP) in single-member districts, Saripolo or Sartori method (Largest remainder, but remainders only for those with no seats) in multi-member districts
Plurality voting refers to electoral systems in which a candidate who polls more than any other counterpart is elected. In systems based on single-member districts, it elects just one member per district and may also be referred to as first-past-the-post (FPTP), single-member plurality (SMP/SMDP), single-choice voting, simple plurality or relative majority. A system which elects multiple winners elected at once with the plurality rule, such as one based on multi-seat districts, is referred to as plurality block voting.
Proportional representation (PR) refers to a type of electoral system under which subgroups of an electorate are reflected proportionately in the elected body. The concept applies mainly to political divisions among voters. The essence of such systems is that all votes cast – or almost all votes cast – contribute to the result and are effectively used to help elect someone – not just a bare plurality or (exclusively) the majority – and that the system produces mixed, balanced representation reflecting how votes are cast.
Party-list proportional representation (list-PR) is a subset of proportional representation electoral systems in which multiple candidates are elected through their position on an electoral list. They can also be used as part of mixed-member electoral systems.
The single transferable vote (STV), sometimes known as proportional ranked choice voting (P-RCV), is a multi-winner electoral system in which each voter casts a single vote in the form of a ranked-choice ballot. Voters have the option to rank candidates, and their vote may be transferred according to alternate preferences if their preferred candidate is eliminated or elected with surplus votes, so that their vote is used to elect someone they prefer over others in the running. STV aims to approach proportional representation based on votes cast in the district where it is used, so that each vote is worth about the same as another.
The additional-member system (AMS) is a mixed electoral system under which most representatives are elected in single-member districts (SMDs), and the other "additional members" are elected to make the seat distribution in the chamber more proportional to the way votes are cast for party lists. It is distinct from parallel voting in that the "additional member" seats are awarded to parties taking into account seats won in SMDs, which is not done under parallel voting.
Mixed-member proportional representation is a mixed electoral system in which votes are cast for both local elections and also for overall party vote tallies, which are used to allocate additional members to produce or deepen overall proportional representation.
First-past-the-post voting is an electoral system wherein voters cast a vote for a single candidate, and the candidate with the most votes wins the election. Analogous systems for multi-winner contests are known as plurality block voting or "block voting" systems; both FPTP and block voting are "plurality" systems in that the winner needs only a plurality of the votes and not an absolute majority. The term first-past-the-post is a metaphor from horse racing of the plurality-voted candidate winning such a race; the electoral system is formally called single-member plurality voting (SMP) when used in single-member districts, and informally called choose-one voting in contrast to ranked voting or score voting.
Parallel voting is a type of mixed electoral system in which representatives are voted into a single chamber using two or more different systems, most often first-past-the-post voting (FPTP) with party-list proportional representation (PR). It is the most common form of mixed member majoritarian representation (MMM), which is why these terms are often used synonymously with each other. In some countries, parallel voting is known as the supplementary member (SM) system, while in academic literature it is sometimes called the superposition method within mixed systems.
In electoral systems, a wasted vote is any vote that does not receive representation in the final election outcome. This includes lost votes for a losing candidate or party, and excess votes for winning candidates in excess of the minimum needed to win. When applied to ranked-vote systems, it includes exhausted votes, votes where none of the candidates the voter ranked are elected. Wasted vote share changes from one election to another depending on voter behavior. Wasted votes can lead to political apathy.
The general ticket, also known as party block voting (PBV) or ticket voting, is a type of block voting in which voters opt for a party, or a team's set list of candidates, and the highest-polling party/team becomes the winner. Unless specifically altered, this electoral system results in the victorious political party receiving 100% of the seats. Rarely used today, the general ticket is usually applied in more than one multi-member district, which theoretically allows regionally strong minority parties to win some seats, but the strongest party nationally still typically wins with a landslide.
A party-list system is a type of electoral system that formally involves political parties in the electoral process, usually to facilitate multi-winner elections. In party-list systems, parties put forward a list of candidates, the party-list who stand for election on one ticket. Voters can usually vote directly for the party-list, but in other systems voters may vote for directly individuals candidates within or across party lists, besides or instead of voting directly for parties.
The multiple non-transferable vote (MNTV) is a group of voting system, in which voters elect several representatives at once, with each voter having more than one vote. MNTV uses multi-member electoral districts or only one district, which contains all voters, which is used to provide at-large representation.
An electoral system or voting system is a set of rules that determine how elections and referendums are conducted and how their results are determined. Electoral systems are used in politics to elect governments, while non-political elections may take place in business, non-profit organisations and informal organisations. These rules govern all aspects of the voting process: when elections occur, who is allowed to vote, who can stand as a candidate, how ballots are marked and cast, how the ballots are counted, how votes translate into the election outcome, limits on campaign spending, and other factors that can affect the result. Political electoral systems are defined by constitutions and electoral laws, are typically conducted by election commissions, and can use multiple types of elections for different offices.
A majoritarian electoral system is an electoral system where the candidate with the most votes takes the seat using the winner-takes-all principle and in this way provides majoritarian representation. However, there are many electoral systems considered majoritarian based on different definitions, including types of at-large majoritarian representation such as block voting or party block voting, but district-based majoritarian systems such as first-past-the-post voting (FPTP/SMP). Where two candidates are in the running, the one with the most votes will have a majority, but where there are three or more candidates, it often happens that no candidate takes a majority of the votes.
Scorporo is a partially compensatory, mixed-member majoritarian electoral system, sometimes referred to as a negative vote transfer system (NVT) whereby a portion of members are elected in single-member districts (SMDs) and a portion are elected from a list. It may be fully defined as a parallel voting system which excludes a portion of the SMD winners' votes in electing the proportional tier, to result in a more proportional outcome. The exclusion of a portion of the SMD winners' votes is what makes scorporo fundamentally different from parallel voting and somewhat closer to mixed member proportional representation, and thereby between the two in terms of proportionality. The system is only known to have been used in Italy and for a portion of the compensatory tier of the National Assembly of Hungary.
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.
The Italian electoral law of 2017, colloquially known by the nickname Rosatellum bis or simply Rosatellum after Ettore Rosato, the Democratic Party (PD) leader in the Chamber of Deputies who first proposed the new law, is a parallel voting system, which acts as a mixed electoral system, with 37% of seats allocated using a first-past-the-post electoral system and 63% using a proportional method, with one round of voting. The Chamber and Senate of the Republic did not differ in the way they allocated the proportional seats, both using the largest remainder method of allocating seats.
The mixed single vote (MSV) or positive vote transfer system (PVT) is a mixed-member electoral system, where voters cast a single vote in an election, which used both for electing a local candidate and as a vote for a party affiliated with that candidate according to the rules of the electoral system. Unlike the more widespread mixed proportional and mixed majoritarian systems where voters cast two votes, split-ticket voting is either not possible or not allowed in MSV.
The mixed ballot transferable vote (MBTV) refers to a type of vote linkage-based mixed-member electoral system where a group of members are elected on local (lower) tier, for example in single-member districts (SMDs). Other members are elected on a compensatory national (upper) tier from a list and voters cast a single ballot where they may indicate their preferences separately.
Mixed member majoritarian representation (MMM) is type of a mixed electoral system combining majoritarian and proportional methods, where the disproportional results of the majoritarian side of the system prevail over the proportional component. Mixed member majoritarian systems are therefore also as a type of semi-proportional representation, and are usually contrasted with mixed-member proportional representation (MMP) which aims to provide proportional representation via additional compensation ("top-up") seats.
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