Coombs' method: Difference between revisions

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{{Short description|Single-winner ranked-choice electoralvoting systemrule}}{{Confused|Combs method|text=the [[Combs method]]}}{{Electoral systems sidebar|expanded=Single-winner}}

{{Electoral systems}}

'''Coombs' method''' is a [[ranked voting systems|ranked voting system]]. Like [[Instant-runoff voting|instant-runoff (IRV-RCV)]], Coombs' method is a [[Sequential elimination method|sequential-loser method]], where the last-place finisher according to one method is eliminated in each round. However, unlike in instant-runoff, each round has electors voting against their least-favorite candidate; the candidate ranked last by the most voters is eliminated.<ref name="Grofman">{{Cite journal |last=Grofman |first=Bernard |last2=Feld |first2=Scott L. |date=2004-12-01 |title=If you like the alternative vote (a.k.a. the instant runoff), then you ought to know about the Coombs rule |url=https://www.sciencedirect.com/science/article/abs/pii/S026137940300060X |journal=Electoral Studies |volume=23 |issue=4 |pages=641–659 |doi=10.1016/j.electstud.2003.08.001 |issn=0261-3794|url-access=subscription }}</ref>

'''Coombs' method''' or the '''Coombs rule'''<ref name=Grofman>Grofman, Bernard, and Scott L. Feld (2004) [https://dx.doi.org/10.1016/j.electstud.2003.08.001 "If you like the alternative vote (a.k.a. the instant runoff), then you ought to know about the Coombs rule,"] ''Electoral Studies'' '''23''':641-59.</ref> is a [[ranked voting systems|ranked voting system]], created by [[Clyde Coombs]], which uses a ballot counting method for [[ranked voting]]. Coombs' method can be thought of as a cross between [[instant-runoff voting]] and [[anti-plurality voting]].

The method satisfiesfails theseveral [[Majorityvoting favoritesystem criterion|majority criterioncriteria]], theincluding [[paretoCondorcet winner criterion]], and the [[|Condorcet's losermajority criterion]], but fails to satisfy both [[later-no-harm]] and [[later-no-helpMonotonicity criterion|later-no-help]]. The method also fails the [[Condorcet criterionmonotonicity]], the [[monotonicityParticipation criterion|participation]], and [[Independence of irrelevantclones alternativescriterion|clone-independence]].<ref>{{cite journal |last=Nurmi |first=Hannu |title=Voting Procedures: A Summary Analysis |journal=British Journal of Political Science |volume=13 |issue=2 |pages=181-208 |publisher=Cambridge University Press |date=1983-04-01 |language=English |url=https://www.cambridge.org/core/journals/british-journal-of-political-science/article/abs/voting-procedures-a-summary-analysis/67C52E7250EB4B88018B22D59CAA6908 |doi=10.1017/S0007123400003215 |access-date=2024-05-19 |url-access=subscription }}</ref><ref>{{cite book |last=Nurmi |first=Hannu |title=Comparing Voting systems |publisher=Springer Dordrecht |series= Theory and Decision Library A |volume=3 |edition=Illustrated |date=2012-12-06 |pages=209 |language=English |url= https://link.springer.com/book/10.1007/978-94-009-3985-1?utm_medium=referral&utm_source=google_books&utm_campaign=3_pier05_buy_print&utm_content=en_08082017 |doi= 10.1007/978-94-009-3985-1 |isbn= 9789400939851}}</ref> However, it does satisfy Black's single-peaked [[Median voter property|median voter criterion]].<ref name="Grofman" />{{rp|at=prop. 2}}

Like instant runoff, Coombs' method candidate elimination and redistribution of votes cast for that candidate until one candidate has a majority of votes. However, unlike instant-runoff, each round eliminates the candidate rated last by the most voters (instead of first by the fewest voters).

== History ==

The method satisfies the [[Majority favorite criterion|majority criterion]], the [[pareto criterion]], and the [[Condorcet loser criterion]], but fails to satisfy both [[later-no-harm]] and [[later-no-help criterion|later-no-help]]. The method also fails the [[Condorcet criterion]], the [[monotonicity criterion]], and [[Independence of irrelevant alternatives]].<ref>{{cite journal |last=Nurmi |first=Hannu |title=Voting Procedures: A Summary Analysis |journal=British Journal of Political Science |volume=13 |issue=2 |pages=181-208 |publisher=Cambridge University Press |date=1983-04-01 |language=English |url=https://www.cambridge.org/core/journals/british-journal-of-political-science/article/abs/voting-procedures-a-summary-analysis/67C52E7250EB4B88018B22D59CAA6908 |doi=10.1017/S0007123400003215 |access-date=2024-05-19 }}</ref><ref>{{cite book |last=Nurmi |first=Hannu |title=Comparing Voting systems |publisher=Springer Dordrecht |series= Theory and Decision Library A |volume=3 |edition=Illustrated |date=2012-12-06 |pages=209 |language=English |url= https://link.springer.com/book/10.1007/978-94-009-3985-1?utm_medium=referral&utm_source=google_books&utm_campaign=3_pier05_buy_print&utm_content=en_08082017 |doi= 10.1007/978-94-009-3985-1 |isbn= 9789400939851}}</ref>

The method was popularized by [[Clyde Coombs]].<ref name="Grofman" /> It was described by [[Edward J. Nanson]] as the "Venetian method"<ref>{{Cite book |last=Royal Society of Victoria (Melbourne |first=Vic ) |url=http://archive.org/details/transactionsproc1719roya |title=Transactions and proceedings of the Royal Society of Victoria .. |date=1864 |publisher=Melbourne : The Society |others=American Museum of Natural History Library}}</ref> (which should not be confused with the [[Republic of Venice]]'s use of [[score voting]] in elections for [[Doge of Venice|Doge]]).

==Procedures==

Each voter rank-orders all of the candidates on their ballot. Otherwise, the candidate ranked last by the largest number ([[plurality (voting)|plurality]]) of voters is eliminated, making each individual round resembleequivalent to [[anti-plurality voting]]. Conversely, under [[instant-runoff voting]], the candidate ranked first (among non-eliminated candidates) by the fewest voters is eliminated.

In some sources, the elimination proceeds regardless of whether any candidate is ranked first by a majority of voters, and the last candidate to be eliminated is the winner.<ref>Pacuit, Eric, [https://plato.stanford.edu/archives/fall2017/entries/voting-methods/ "Voting Methods"], ''The Stanford Encyclopedia of Philosophy'' (Fall 2017 Edition), Edward N. Zalta (ed.)</ref> This variant of the method can result in a different winner than the former one (unlike in instant-runoff voting, where checking to see if any candidate is ranked first by a majority of voters is only a shortcut that does not affect the outcome).

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== In practice ==

The voting rounds used in the [[reality television]] program ''[[Survivor (TV series)|Survivor]]'' could be considered a variation of Coombs' method, but with sequential voting rounds. Everyone votes for one candidate they support for elimination each round, and the candidate with a plurality of that vote is eliminated. A strategy difference is that sequential rounds of voting means the elimination choice is fixed in a ranked ballot Coombs' method until that candidate is eliminated.

=== Potential for strategic voting ===

Like [[anti-plurality voting]], Coombs' rule is extremely vulnerable to strategic voting. As a result, it is mostmore often consideredused as an example of a [[Pathological (mathematics)|pathological]] voting rule rather than in anya serious useproposal.<ref name=":0" /> The equilibrium position for Coombs' method is extremely sensitive to [[Instant-runoffExhausted voting#Invalid ballots and incomplete ballotsballot|incomplete ballots]], [[Tactical manipulation of runoff voting#Compromise|compromising]], [[Tactical manipulation of runoff voting#Push over|push-over]], and [[strategic nomination|teaming]], andbecause the vast majority of voters' effects on the election come from how they fill out the bottom of their ballots.<ref name=":0">[http://www.accuratedemocracy.com/l_data.htm "Data on Manipulability"]</ref> As a result, voters have a strong incentive to rate the strongest candidates last to defeat them in earlier rounds.<ref>{{Cite journal |last=Smith |first=Warren D. |date=12 July 2006 |title=Descriptions of single-winner voting systems |url=http://www.9mail.de/m-schulze/votedesc.pdf |journal=Voting Systems}}</ref>

This results in a [[Keynesian beauty contest|Keynesian beauty pageant]] that is extremely sensitive to minor variations in the perceived strengths of candidates.

==See also==

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{{voting systems}}

[[Category:Single-winner electoral systems]]

[[Category:Non-monotonic electoral systems]]

[[Category:Preferential electoral systems]]