Independence of irrelevant alternatives: Difference between revisions

* '''Independence of Smith-dominated alternatives''' (ISDA), also sometimes called Smith-IIA (Smith-Independence of Irrelevant Alternatives), which 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 [[Smith set]]. ISDA implies [[Smith criterion|Smith]] and thus [[Condorcet criterion|Condorcet]], since logically speaking, if an ISDA-passing method's winner were not in the Smith set, eliminating everyone outside of the Smith set would have to change the winner. Some Condorcet methods (e.g. [[Schulze method|Schulze]]) satisfy ISDA. Any Condorcet method that starts by eliminating everyone outside the Smith set passes ISDA. Satisfaction of ISDA can sometimes make understanding a voting method or finding the winner easier; see the [https://en.wikipedia.org/wiki/Schulze_method#Ties_and_alternative_implementations Schwartz set heuristic] for Schulze for an example. ISDA is a natural extension of the Smith criterion because it can be phrased as analogous to the following property implied by the Condorcet criterion: "if there is a [[Condorcet winner]], and candidates are added or removed to the election who are pairwise beaten by the [[Condorcet winner]], then the winner does not change". (ISDA's analogous phrasing is "if there is a Smith set, and candidates are added or removed to the election who are pairwise defeated by everyone in the Smith set, then the winner does not change." Note that unlike the Condorcet winner, the Smith set always exists.) ISDA is incompatible with IIA, since ISDA implies [[Majority criterion|majority]] and majority is incompatible with IIA. Given Schulze's multi-winner generalization of the Smith set (see the "Multi-winner generalizations" section of the [[Smith criterion]] article), an analogous extension of ISDA for the multi-winner case might be "if candidates not in any groups of candidates guaranteed seats by Schulze's multi winner Smith criterion drop out or enter the race, this shouldn't change the seat guarantees given to those same groups."

* '''Independence of covered alternatives''' which 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 also implies 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.

* '''Independence of Pareto-dominated alternatives''' (IPDA), which says that if one option (X) wins an election, and a new alternative (Y) is added, X will win the election if Y is Pareto-dominated. An alternative W is Pareto-dominated if there exists some other alternative Z so that no voter ranks W ahead of Z and at least one voter ranks Z ahead of W.