Later-no-harm criterion: Difference between revisions
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Added probabilistic definition that follows from Woodall's definition of harm in the 1994 paper
(Linking to Douglas Woodall's papers from 1994 and 1997, one of which was published in Voting Matters) |
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{{Wikipedia}}
The "'''later-no-harm criterion'''" criterion (sometimes referred to as "'''LNHarm'''"<ref>{{Cite web |title=
== Definition ==
Here's a definition of the later-no-harm criterion
{{definition|A voter giving an additional ranking or positive rating to a less-preferred candidate cannot cause a more-preferred candidate to lose.}}
For probabilistic methods, the later-no-harm criterion is:
{{definition|A voter giving an additional ranking or positive rating to a less-preferred candidate cannot reduce the probability that a more-preferred candidate wins.}}
== Complying Methods ==
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<p>'''Later-no-harm''' (usually LNH, but sometimes LNHa or LNHarm to avoid confusion with [[Later-no-help]]) is satisfied by [[IRV|Instant Runoff Voting]], [[Minmax|Minmax(pairwise opposition)]], and Douglas Woodall's [[Descending Solid Coalitions]] method. It is trivially satisfied by [[Plurality voting|First-Preference Plurality]] and [[Random Ballot]], since those methods do not usually regard lower preferences. Virtually every other method fails this criterion.</p>
<p>'''Later-no-harm''' is incompatible with the [[Condorcet Criterion|Condorcet criterion]].<ref name="Woodall97" /></p>
== Example ==
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=== Preventing compromise ===
This criterion is equivalent to the criterion that the system is non-compromising in that it will never elect a compromise (i.e. a [[Utilitarian winner]] or [[Condorcet winner]].) This is not universally desired so it cannot be claimed that this
=== Not reflecting voter preferences ===
One argument against LNH is that it can result in arbitrary changes in election outcomes based on voter preferences.<ref>{{Cite web|url=https://medium.com/@ClayShentrup/later-no-harm-72c44e145510|title=Later-no-harm|last=Shentrup|first=Clay|date=2020-01-02|website=Medium|language=en|access-date=2020-04-30}}</ref>
A counterargument would be that the example given involves two major candidates, L and R, and because the voters' [[pairwise preference]]<nowiki/>s between the two didn't change (rather, their preference between L/R and C changed), no change should occur in who was the better of the two. This argument is an example of how those against LNH are often in favor of [[rated method]]<nowiki/>s, where the strength of each of the voter's pairwise preferences are connected (i.e. the fact that some voters increased their support for a major candidate in relation to an irrelevant candidate is argued to mean that their strength of preference between the two major candidates ought to be weakened), rather than [[ranked method]]<nowiki/>s, which are often based on the idea that a voter's pairwise preferences are independent and maximal (i.e. [[:Category:Pairwise counting-based voting methods]]).
== Notes ==
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== References ==
<
[[Category:Voting system criteria]]
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