Instant-runoff voting: Difference between revisions
Bolded the "RCV" part at the top, added in section on equal-ranking, and a section on how IRV can ignore ballot information to the detriment of a majority,
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(Bolded the "RCV" part at the top, added in section on equal-ranking, and a section on how IRV can ignore ballot information to the detriment of a majority,) |
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When the [[single transferable vote]] (STV) [[voting system]] is applied to a single-winner election it is sometimes called '''instant-runoff voting''' (IRV), as it is much like holding a series of [[runoff voting|runoff]] elections in which the lowest polling candidate is eliminated in each round until someone receives [[simple majority|majority]] vote. IRV is often considered independently of multi-winner STV because it is simpler and because it is the most widely advocated electoral reform in the United States.
Outside the USA, IRV is known as the '''[[Alternative Vote]]''', '''[[preferential voting]]''', '''single-winner STV''', or the '''[[Thomas Hare|Hare]] System''', though there is room for confusion with some of these terms since they can also refer to STV in general. In the US, IRV is also known as '''Ranked Choice Voting''' ('''RCV'''), a term useful for describing the voter's experience as well as the appearance of the ballot.
Instant-Runoff Voting was invented around 1870 by American architect [[William Robert Ware]]. Ware was not a mathematician, thus never subjected his election method to any rigorous analysis. He evidently based IRV on the single winner outcome of the [[Single Transferable Vote]] or STV developed in 1855 originally by [[Carl Andrae]] in [[Denmark]]. It was introduced into [[England]] in 1857 by the [[barrister]] [[Thomas Hare]], where it earned public praise from [[John Stuart Mill]], an English philosopher, member of parliament, and employee of the [[British East India Company|East India Company]].
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** [http://alumnus.caltech.edu/~seppley/MAM%20procedure%20definition.htm Random Voter Hierarchy (RVH)]: Randomly determine a strict ordering of the candidates and when selecting a candidate to eliminate, pick one based on this strict ordering.
*** Similar to random elimination, but with many nice properties not found with random elimination
=== '''Variants''' ===
IRV can be done with equal ranking allowed. The two main ways of doing this are either fractional (split the voter's ballot equally between all of their highest-ranked candidates that are ranked equally (3 candidates ranked 1st each get 1/3rd of a vote)), or whole votes (give each highest-equally-ranked candidate one vote (3 candidates get 1 vote each and 3 votes total)).
With whole votes equal-ranking, there are two ways to find a winner (which give the same result in standard IRV but differ for whole votes): either eliminate candidates until only two remain, and declare the one with more votes the winner, or eliminate candidates until one or more candidates are supported by a majority of active ballots, and then elect the candidate with the largest majority. Some have argued<ref>[https://www.reddit.com/r/EndFPTP/comments/e6bt6s/proportionality_failure_in_stv_with_equalranks/f9s5yno/?context=3]</ref> that in order to limit opportunities for pushover strategy with whole votes, a ballot that equally ranks candidates should be allowed to help those candidates win, but not prevent those candidates from getting eliminated.
== Where IRV is used ==
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In IRV, the compromise (choice C) is eliminated immediately. Choice B is elected, giving severely lower total satisfaction amongst voters than choice C.
'''Failure to count the ballots in a way most favorable to the voters'''
{| border="1"
!26% of voters
!25% of voters
!49% of voters
|-
|1. A
|1. C
|1. D
|-
|2. B
|2. B
|
|}
Here, a majority is split between two candidates as their 1st choice, but can unanimously agree on a third candidate as their 2nd choice. IRV instantly eliminates the majority's 2nd choice for having no 1st choice votes, then eliminates C, and then elects D. Yet a majority of voters preferred a different outcome.
===Logistical issues===
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