Proportional representation: Difference between revisions
→Proportional Representation Criteria
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== Proportional Representation Criteria==
Since the standard definitions of Proportional Representation do not apply to nearly all modern systems it has become common to define proportional representation in terms of passing some sort of criteria. There is no consensus on which criteria need to be passed for a parliament to be said to be proportional, though most can agree that a voting method that passes one of the [[weak forms of PSC]] (several of which are listed here) is at least semi-proportional.
=== [[Proportionality for Solid Coalitions]] Criterion ===
PSC: If a sufficiently-sized group (generally at least a Droop or Hare quota) prefer a set of candidates above all others, do at least a proportional number (being the number of quotas the group comprises rounded down to the nearest integer) of candidates from that set (supposing there are enough of them) get elected?
===Proportional (Ideological) Representation Criterion===
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Whenever a group of voters gives max support to their favoured candidates and min support to every other candidate, at least one seat less than the portion of seats in that district corresponding to the portion of seats that that group makes up{{clarify}} is expected to be won by those candidates.
One of the effects of this property is that if all voters vote solely on party lines (max support to everyone in your party and min support to everyone outside of it), then the proportion of popular vote for candidates associated to parties is roughly equal to the proportion of members elected for each party. This is a weak form of PSC identical to “Partisan Proportionality” in the case that all groups large enough to expect a winning candidate have a party which they identify with and their candidate belongs to.
===Partisan Proportionality Criterion===
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Whenever more than a [[Hare Quota]] of the voters gives max support to a single candidate and min support to every other candidate, that candidate is guaranteed to win regardless of how any of the other voters vote.
Any method that passes the Proportional Representation Criterion also passes the Hare Quota Criterion. This is a very weak form of PSC.
===Winner Independent Proportionality Criterion===
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===Combined Independent Proportionality Criterion===
The winner set must be proportional even if some losing candidates were disqualified, scores for some losing candidates were reduced, and/or the scores for some winning candidates were increased. That is, if at least n quotas of ballots approve the same set of candidates, but there is partial disagreement on some candidates outside of that set, m of whom were elected, then at least n-m candidates in the set must be elected. (If 2 quotas approve ABCD, 2 quotas approve ABCDE, the standard PR criterion would require 2 of ABCD to be elected, whereas this criterion would require 4 of ABCDE to be elected.) These last two criteria are related to PSC.
== Proportional Systems ==
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