Independence of irrelevant alternatives

In voting systems, independence of irrelevant alternatives refers to the property some voting systems have that, if one option (X) wins the election, and a new alternative (Y) is added, only X or Y will win the election.

A less strict property is sometimes called local independence of irrelevant alternatives. It 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.

The family of voting systems called Condorcet's method satisfy this criterion.

None of the Borda count, Coombs' method or Instant-runoff voting meet either criterion.

Some text of this article is derived with permission from http://condorcet.org/emr/criteria.shtml