Stable winner set: Difference between revisions
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A stable winner set is a requirement on a winner set
Given a winner set S of K winners, another winner set S' containing K’ winners blocks S
Where V(S,S’) is the number of voters who strictly prefer S’ to S and n is the number of voters.
A winner set is stable if no replacement set blocks it.
There are a few points which are important to note:
# In most cases K’ < K. This means that the definition of a stable winner set is that a subgroup of the population cannot be more happy with less winners given the relavant size comparison of the Ks and the group. This relates to how PR methods attempt to maximize voters' representation by electing those who sizeable subgroups each strictly prefer, rather than only those who only majorities or pluralities can agree on.
# There can be more than one stable winner set. The group of all stable winner sets is referred to as ''the core''.
#The term "strictly prefer"
#The formula V(S,S’)/n >= K’/K is analogous to a (Hare) quota; formulas analagous to other quotas may be used instead.
==Relation to Proportional Representation==
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It can be argued that even though {C, D} doesn't include any voter's 1st choice candidate, whereas {A,B} includes a 1st choice candidate of every voter, {C,D} is a better solution because it maximizes all voters' satisfaction with the overall set of winners. In essence, 1 point of utility is lost when comparing a voter's favorite candidate in each set but 9 points are gained for their 2nd-favorite candidate in each set. This form of analysis has been done with Condorcet PR methods before, though not while also discussing cardinal utility:<blockquote>Lifting preferences from candidates to committees is achieved through what we call ''f''-preferences. A given voter has an ''f''-preference for one possible committee ''A'' over another, ''B'', if the voter prefers ''A'' to ''B'' when considering in each committee '''only''' the ''f'' candidates most preferred by that voter. For example, a voter has a 1-preference for committee ''A'' over committee ''B'' if the voter' favorite candidate in committee ''A'' is preferred by that voter over the voter's favorite candidate in committee ''B''. The voter has a 2-preference for committee ''A'' over committee ''B'' if the two favorite candidates on committee 1 are preferred over the two favorite candidates on committee ''B''.<ref name=":0">https://civs.cs.cornell.edu/proportional.html</ref></blockquote>Note that under the "voter prefers the set with more of their highest-preferred candidates" definition, {A,B} would be the stable set here. This definition makes stable sets appear to become more analagous to a Smith-efficient Condorcet PR method, such as [[Schulze STV]].
(It is possible to create various hybrids of the two definitions
== Droop Version==
If the formula V(S,S’)/n >= K’/K is modified to instead be V(S,S’)/n >= K’/'''(K+1)''', then this makes stable sets' definition of proportionality become more similar to other definitions of PR that use Droop [[Quota]]s (or more specifically, Hagenbach-Bischoff Quotas) rather than Hare [[Quota]]s.
This definition is more restrictive and as such has a number of undesirable situations where it eliminates all winner sets from being stable. This can happen even in super simple examples, e.g., two voters, one likes A, the other likes B, and one candidate to elect -- neither {A} nor {B} is stable.
49% vote A5 B3 C0▼
The biggest issue when using the Droop stability definition arises in the single-winner case, which is that the Score winner may no longer be stable. Consider the following example:
2% vote A5 B3 C5▼
▲49% vote A5 B3 C0
▲2% vote A5 B3 C5
49% vote A0 B3 C5
B is the Score/Utilitarian winner and is stable under Hare but not Droop.
Also, stable sets can have this "quota" computed based solely on voters who have preferences between any pair of sets that are being compared, so that in a 2-winner Approval Voting election with 67 A 33 B 10 C, the quota when looking at matchups between sets including either or both A and B is only computed off of at most the 100 voters that have preferences between them, rather than all 110. This would fix some but not all of the issues with this definition.
Using Droop Quotas and this "only voters with preferences between the relevant sets are used to compute the quota" trick makes stable sets become a Smith-efficient Condorcet method in the single-winner case
Interestingly, the two varying modes of deciding which set a voter prefers in each pairwise matchup (evaluate a voter's preference between sets as based either on
==Further Reading==
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