Proportional Representation (PR) describes various multi-winner voting systems which try to ensure that the proportional support gained by different groups is accurately reflected in the election result. Proportional representation is also used to describe this (intended) effect.
In practice this usually involves ensuring that political parties in parliament or legislative assemblies receive a number of seats (approximately) proportional to the percentage of vote they received. This is known as party-list proportional representation. Another kind of electoral system that strives to achieve proportional representation but which does not rely on the existence of political parties is the single transferable vote (STV). Some electoral systems, such as the single non-transferable vote and cumulative voting are sometimes categorized as "semi-proportional".
Electoral systems that do not result in proportional representation are known as majoritarian systems. These include first-past-the-post (plurality), runoff voting (majority), the alternative vote and the bloc vote. In these systems, parties can receive seat numbers that bear little or no relationship to the national percentages they received in parliament. This is called disproportionality, and can be measured with the Gallagher Index, which takes the square root of half the sum of the squares of the difference between percent of vote and percent of seats for each party.
The district or constituency magnitude of a system (i.e. the number of seats in a constituency) plays a vital role in determining how proportional an electoral system can be. When using proportional systems, the greater the number of seats in a district or constituency, the more proportional it can be. Any system with single-member districts is by necessity majoritarian at the district or constituency level. However, district or constituency borders may be gerrymandered to create "majority-minority" districts or constituencies where a group of voters in the minority system-wide form the majority in a particular district or constituency, thus allowing a simulation of proportionality system-wide.
However, multiple-member districts do not ensure that an electoral system will be proportional. The bloc vote can result in "super-majoritarian" results in which, in addition to the normal disproportionality of single-member majoritarian systems, geographical variations that could create majority-minority districts become subsumed into the larger districts.
Proportional representation is unfamiliar to most citizens of the United States, but it is actually a much more common system of voting than first-past-the-post. In general, first-past-the-post is only used in former British colonies, but a form of proportional representation known as the mixed member system is now being used in the United Kingdom to elect the members of the Scottish Parliament and the Welsh National Assembly. Although once an unknown system, Proportional representation is now gaining popularity in Canada with three provinces: British Columbia, Ontario and Quebec, now committed to abolishing first past the post, at the federal level, a Parliamentary Committee is now exploring the issue and many Canadians are showing interest in adopting a form of Proportional Representation. Political analysts point out the fact that the current attitude and sequence of events is very similar to what happened in the New Zealand when, New Zealand opted for Mixed Member Proportional Representation and the analysts conclude Canada is heading down the same route. All of the members of the European Parliament, or MEPs, including those elected from constituencies in Britain, are elected by proportional representation. Proportional representation is also used in many European countries.
Proportional representation does have some history in the United States. Many cities, including New York, once used it for their city councils as a way to break up the Democratic Party monopolies on elective office. In Cincinnati, Ohio, proportional representation was adopted in 1925 to get rid of a Republican party machine (the Republicans successfully overturned proportional representation in 1957).
Some electoral systems incorporate additional features to ensure absolutely accurate or more comprehensive representation, based on gender or minority status (like ethnicity). Note that features such as this are not strictly part of proportional representation; depending on what kind of PR is used, people tend to be already represented proportionally according to these standards without such additional rules.
See Two-Party System: Arguments For and Against for a list of perceived advantages of proportional representation.
 Further reading
- John Hickman and Chris Little. "Seat/Vote Proportionality in Romanian and Spanish Parliamentary Elections" Journal of Southern Europe and the Balkans Vol. 2, No. 2, November 2000safd
- See the Proportional Representation Library http://www.mtholyoke.edu/acad/polit/damy/prlib.htm
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