Semi-proportional representation

Single-vote systems

Election systems in which a party can achieve its due share of seats (proportionality) only by coordinating its voters are usually considered to be semi-proportional. They are not non-proportional or majoritarian, since in the perfect case the outcome will be proportional, but they are not proportional either, since the perfect case is not guaranteed without coordination. Such systems include the single non-transferable vote and cumulative voting, both of which are commonly used to achieve approximately-proportional outcomes while maintaining simplicity and reducing the cost of election administration. Under these systems, parties often coordinate voters by limiting the size of the party slate, or by using complex vote management schemes where voters are asked to randomize which candidate(s) they support.

These systems are notable for the absence of an ordered electoral list. Candidates may coordinate their campaigns, and present or be presented as agents of a party, but voters may choose to support one candidate among the said group but not the others (that is, panachage is permitted).

Single transferable vote

Many writers consider the single transferable vote to be a semi-proportional system because of its substantial favoritism towards major parties, generally caused by a combination of the Droop quota in small districts, as well as the substantial degree of vote management involved when there are exhausted ballots. On the other hand, some authors describe it as a proportional system, on the grounds that it is theoretically weakly proportional in the limit of infinitely-large constituencies. However, it is worth noting that STV is only proportional for solid coalitions, i.e. if voters rank candidates first by party and only then by candidate. As such, the proportionality of STV breaks down if voters are split across party lines or choose to support candidates of different parties.

A major complication with proportionality under STV is the need for constituencies; small constituencies are strongly disproportional, but large constituencies make it difficult or impossible for voters to rank large numbers of candidates, turning the election into a de facto open list PR system, particularly where voters lack any meaningful information about the candidates on their ballot.

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 received 45.2% of the seats with just 36.1% of the first preference votes. In the 2020 Irish general election, the Labour Party received 50% more votes than the Social Democrats, but both parties won the same number of seats. Ireland uses districts of 3-7 members.

Similarly, the 1998 Northern Ireland elections resulted in the Ulster Unionists winning more seats than the Social Democratic and Labour Party with a smaller share of the vote.

The proportionality of STV can be controversial, especially in close elections like the 1981 election in Malta. In this election, the Maltese Labour party won a majority of seats despite the Nationalist Party winning a majority of first preference votes. This caused a constitutional crisis, leading to a provision to provide bonus seats in case of disproportional results. These bonus seats were needed in 1987, 1996, and 2008 to prevent further electoral inversions.

The degree of proportionality nationwide is strongly related to the number of seats to be filled in each constituency. In a three-seat constituency using the Droop quota, a full quarter of the vote is wasted. In a nine-seat constituency, only a tenth of the vote is wasted, and a party needs only 10% of the vote to win a seat. Consequently, the best proportionality is achieved when there are a large number of representatives per constituency. The Hare quota is theoretically unbiased, allowing some of the errors in apportionment to cancel out if voters across the whole country. However, it also increases the vulnerability of STV to vote management by large parties, allowing them to win the same number of seats they would have won under Droop.

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. Such a system is used in Hungary in local elections. 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. It has remained in use in Italy, as well as seeing some use in San Marino, Greece, and France.

The simplest mechanism to reinforce major parties in PR system is to severely restrict the number of seats per electoral district, which increases the Droop quota (the number of votes needed to be guaranteed a seat).

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.

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