Coombs' method: Difference between revisions

Revision as of 23:02, 6 February 2024

Revision as of 04:48, 9 February 2024

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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 resemble [[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).

==An example==

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=== Potential for strategic voting ===

Like [[anti-plurality voting]], Coombs' rule is extremely vulnerable to strategic voting. As a result, it is most often considered as an example of a [[Pathological (mathematics)|pathological]] voting rule rather than in any serious use.<ref name=":0" />. Coombs' method is extremely sensitive to [[Instant-runoff voting#Invalid ballots and incomplete ballots|incomplete ballots]], [[Tactical manipulation of runoff voting#Compromise|compromising]], [[Tactical manipulation of runoff voting#Push over|push-over]], and [[strategic nomination|teaming]], and 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.

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