Smith set

In voting systems, the Smith set, named after John H. Smith, is the smallest non-empty set of candidates in a particular election such that each member defeats every other candidate outside the set in a pairwise election. The Smith set provides one standard of optimal choice for an election outcome. Voting systems that always elect a candidate from the Smith set pass the Smith criterion and are said to be "Smith-efficient".

A set of candidates where every member of the set pair-wise defeats every member outside of the set is known as a dominating set.

Properties

• The Smith set always exists and is well-defined. There is only one smallest dominating set since dominating sets are nested, non-empty, and the set of candidates is finite.
• The Smith set can have more than one candidate, either because of pair-wise ties or because of cycles, such as in Condorcet's paradox.
• The Condorcet winner, if one exists, is the sole member of the Smith set. If weak Condorcet winners exist, they are in the Smith set.

Schwartz set comparison

The Schwartz set is closely related to and is always a subset of the Smith set. The Smith set is larger if and only if a candidate in the Schwartz set has a pair-wise tie with a candidate that is not in the Schwartz set.

The Smith set can be constructed from the Schwartz set by repeatedly adding two types of candidates until no more such candidates exist outside the set:

• candidates that have pair-wise ties with candidates in the set,
• candidates that defeat a candidate in the set.

Note that candidates of the second type can only exist after candidates of the first type have been added.

Alternative formulation

Any binary relation R on a set A can generate a natural partial order on the R-cycle equivalence classes of set A, so that xRy implies [x] ≥ [y].

When R is the Beats-or-Ties binary relation on the set of candidates defined by x Beats-or-Ties y if and only if x pair-wise defeats or ties y, then the resulting partial order is the beat-or-tie order which is a total order. The Smith set is the maximal element of the beat-or-tie order.

Algorithms

The Smith set can be calculated with the Floyd–Warshall algorithm in time Θ(n³). It can also be calculated using a version of Kosaraju's algorithm or Tarjan's algorithm in time Θ(n²).

See also

• Condorcet criterion
• Condorcet method
• Preorder
• Partial order

References

• In an analysis of serial decision making based on majority rule, describes the Smith set and the Schwartz set.
• Introduces a version of a generalized Condorcet Criterion that is satisfied when pairwise elections are based on simple majority choice, and for any dominating set, any candidate in the set is collectively preferred to any candidate not in the set. But Smith does not discuss the idea of a smallest dominating set.
• Narrows Smith's generalized Condorcet Criterion to the smallest dominating set and calls it Smith's Condorcet Principle.
• Discusses the Smith set (named GETCHA) and the Schwartz set (named GOTCHA) as possible standards for optimal collective choice.

External links

• Example algorithms to calculate the Smith set