Proportional representation: Difference between revisions

'''Proportional Representation''' ('''PR''') is a measure of the outcome of an election where there are multiple parties and multiple member are election. It is one of many [[types of representation]] in a [[W:Representative Governmentgovernment|representative government]].

In practice this usually involves ensuring that [[politicalW:Political party|political parties]] in parliament or legislative assemblies receive a number of seats (approximately) proportional to the percentage of vote they received through a [[Partisan Systemsystems|partisan system]]. The simplestOne system which achieves high levels of Proportionalproportional Representationrepresentation is the [[Party-list Listproportional Systemrepresentation| 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".

There are several metrics which are used to define Proportional Representation explicitly. The mostA well -accepted form is the [[W:Gallagher index|Gallagher index]]. As such, itwhich measures the difference between the percentage of votes each party gets and the percentage of seats each party gets in the resulting legislature, and it also measures this disproportionality from all parties ''collectively'' in any one given election. That collective disproportionality from the election is given a precise score, which can then be used in comparing various levels of proportionality among various elections from various [[W:Electoral systems|electoral systems]].

[[W:Michael Gallagher (academic)|Michael Gallagher]], who created the index, referred to it as a "least squares index", inspired by the residual sum of squares used in the method of least squares. The index is therefore commonly abbreviated as "LSq" even though the measured allocation is not necessarily a least squares fit. The Gallagher index is computed by taking the [[W:Square root|square root]] of half the [[summationW:Summation|sum]] of the squares of the difference between percent of votes (<math>V_i</math>) and percent of seats (<math>S_i</math>) for each of the political parties (<math>i=1,\ldots,n</math>).