Uncovered set

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The uncovered set is defined for a set of rank-order preferences. Usually, the uncovered set is defined only for situations without pairwise ties. When there are no pairwise ties, then the uncovered set is identical to the set called Fishburn winners:

Select the candidate or candidates that are not Fishburn losers. A candidate i is a Fishburn loser if there is some other candidate j such that every candidate that pairwise beats j also pairwise beats i and there is at least one candidate that pairwise beats i but does not pairwise beat j.

An equivalent definition is that it is the set of every candidate X so that for any Y not in the set, X either beats Y pairwise or X beats someone who beats Y (i.e. X indirectly pairwise beats Y).[1] In this sense, it is related to the concept of a beatpath.

Another definition is:

An alternative a is said to cover alternative b whenever every alternative dominated by b is also dominated by a.[2]

Yet another definition:

The uncovered set is the set of all outcomes x such that there is no outcome beating x and all the outcomes that x beats.[3]

When there are pairwise ties, a likely generalized (yet equivalent when there are no pairwise ties) definition is:

In voting systems, the Landau set (or uncovered set, or Fishburn set) is the set of candidates x such that for every other candidate z, there is some candidate y (possibly the same as x or z) such that y is not preferred to x and z is not preferred to y.

The uncovered set is a nonempty subset of the Smith set. The reason is that every candidate in the Smith set is preferred to every candidate not in the Smith set, therefore each candidate in the Smith set can be considered a candidate x and be their own candidate y; since a candidate can't be preferred to themselves (y is not preferred to x), and since candidates in the Smith set being preferred to every candidate not in the Smith set implies that candidates not in the Smith set are not preferred to candidates in the Smith set (z is not preferred to y), the uncovered set must be a subset of the Smith set.

Example[edit | edit source]

Suppose the following pairwise preferences exist between four candidates (v, x, y, z) (table organized by Copeland ranking):

x y v z Copeland score
x --- Win Lose Win (2-1)=1
y Lose --- Win Win (2-1)=1
v Win Lose --- Lose (1-2)=-1
z Lose Lose Win --- (1-2)=-1

Notice that the Smith set includes all candidates (this can be seen by observing that there is a beatpath of x>y>z>v>x, or alternatively by observing that no matter how many candidates you look at from top to bottom, there is still some candidate outside of the group being looked at that one of the candidates in the group lose or tie to). But the uncovered set is all candidates except z; this is because y>z and all candidates who beat y (just x) also beat z. [4] (Notice that the Copeland set is even smaller; it is just x and y).

An alternative way of understanding the uncovered set in this example is to show the size of the smallest-size beatpath from each candidate to another, if one exists (if x>y is 1 here, this means x pairwise beats y. If it's 2, it means x pairwise beats someone who pairwise beats y, etc.). Any candidate with a smallest-size beatpath of 3 or more to another candidate is not in the uncovered set:

Size of smallest-size beatpath

between each pair of candidates

x y v z
x --- 1 Lose 1
y 2 --- 1 1
v 1 2 --- 2
z 2 3 1 ---

Notice that all candidates except z have beatpaths of size 1 or 2, whereas z>y is (z has a smallest beatpath to y of) 3 steps (z>v>x>y), therefore z is not in the uncovered set.

Notes[edit | edit source]

The uncovered set can be thought of as requiring its candidates to have a two-step beatpath to every candidate not in the uncovered set. The Smith set requires a one-step beatpath (i.e. of at most two candidates, a direct pairwise victory).

Independence of covered alternatives says that if one option (X) wins an election, and a new alternative (Y) is added, X will win the election if Y is not in the uncovered set. Independence of covered alternatives implies Independence of Smith-dominated Alternatives (since independence of covered alternatives implies that one can eliminate everyone outside of the uncovered set without changing the winner, and the uncovered set is a subset of the Smith set, therefore eliminating everyone outside of the Smith set also can't change the winner), which further implies Smith and thus Condorcet. If a method is independent of covered alternatives, then the method fails monotonicity if perfect ties can always be broken in favor of a choice W by using ballots ranking W first.

The Banks set[5] (the set of candidates who could win a sequential comparison contest for at least one ordering of candidates when voters are strategic), Copeland set (set of candidates with the highest Copeland score), and Schattschneider set are all subsets of the uncovered set. [6]

The Banks set is a subset of the Smith set because when all but one candidate in the Smith set has been eliminated in a sequential comparison election, the remaining Smith candidate is guaranteed to pairwise beat all other remaining candidates, since they are all non-Smith candidates, and thus can't be eliminated from that point onwards, meaning they will be the final remaining candidate and thus win.

The Dutta set (also known as Dutta's minimal covering set) is the set of all candidates such that when any other candidate is added, that candidate is covered in the resulting set. It is a subset of the Smith set because all candidates in the Smith set cover (i.e. have a one-step beatpath, direct pairwise victory) all candidates not in the Smith set.

One way that has been suggested to find the uncovered set is:

This suggests the use of the outranking [pairwise comparison] matrix and its square to identify the uncovered set (Banks, 1985):

T = U + U2

where U [is] the tournament matrix. The alternatives represented by rows in T where all non-diagonal entries are non-zero form the uncovered set.[7]

(The square of a matrix can be found using matrix multiplication; here is a video explaining how to do so. The pairwise matrix and its squared matrix can be added together using matrix addition.)

References[edit | edit source]

  1. Munagala, Kamesh; Wang, Kangning (2019-05-04). "Improved Metric Distortion for Deterministic Social Choice Rules". arXiv.org. p. 5. doi:10.1145/3328526.3329550. Retrieved 2020-03-13.
  2. Endriss, U. "Handbook of Computational Social Choice" (PDF). The Reasoner. 2 (10): 57. ISSN 1757-0522. Retrieved 2020-03-13.
  3. https://www.jstor.org/stable/41105997?seq=1
  4. https://economics.stackexchange.com/a/27691
  5. http://spia.uga.edu/faculty_pages/dougherk/svt_13_multi_dimensions2.pdf "The Banks set (BS) is the set of alternatives resulting from strategic voting in a successive elimination procedure"
  6. Seising, R. (2009). Views on Fuzzy Sets and Systems from Different Perspectives: Philosophy and Logic, Criticisms and Applications. Studies in Fuzziness and Soft Computing. Springer Berlin Heidelberg. p. 350. ISBN 978-3-540-93802-6. Retrieved 2020-03-13.
  7. Kilgour, D (2010). Handbook of group decision and negotiation (PDF). Dordrecht New York: Springer. p. 176. ISBN 978-90-481-9097-3. OCLC 668097926.