Sequential elimination method
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The sequential elimination methods are a class of
voting systems that repeatedly eliminate the last-place finisher of another voting method until a single candidate remains.[1] The method used to determine the loser is called the base method. Common are the two-round system, instant-runoff voting, and systems where parties nominate candidates in partisan primaries
.
Baldwin's method is a sequential loser method based on the Borda count.[2]
Properties
Proofs of criterion compliance for loser-elimination methods often use mathematical induction, and so can be easier than proving such compliance for other method types. For instance, if the base method passes the majority criterion, a sequential loser-elimination method based on it will pass mutual majority. Loser-elimination methods are also not much harder to explain than their base methods.[2]
However, loser-elimination methods often fail monotonicity due to chaotic effects (sensitivity to initial conditions): the order in which candidates are eliminated can create erratic behavior.[1]
If the base method passes
independence from the weakest alternative, the loser-elimination method is equivalent to the base method.[1]
In other words, methods that are immune to weak spoilers are already "their own" elimination methods, because eliminating the weakest candidate does not affect the winner.
If the base method satisfies a criterion for a single candidate (e.g. the
Condorcet criterion), then a sequential loser method satisfies the corresponding set criterion (e.g. the mutual majority criterion or the Smith criterion), so long as eliminating a candidate can't remove another candidate from the set in question. This is because when all but one of the candidates of the set have been eliminated, the single-candidate criterion applies to the remaining candidate.[1]
References
- ^ ISBN 978-1-4503-7841-3.
- ^ a b Bag, PK; Sabourian, H; Winter, E. "Sequential Elimination vs. Instantaneous Voting" (PDF). Mimeo.
Further reading
- Chowdhury, Subhasish M.; Kim, Sang-Hyun (July 2017). "'Small, yet Beautiful': Reconsidering the optimal design of multi-winner contests" (PDF). Games and Economic Behavior. 104: 486–493. S2CID 43411835.
- Klunover, Doron (December 2023). "Bureaucracy and labor market inefficiency: A contest model". European Journal of Political Economy. 80: 102472. S2CID 262024392.