Independence of irrelevant alternatives: Difference between revisions

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 In [[voting system]]s, '''independence of irrelevant alternatives''' is 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.
-Most [[Condorcet method]]s fail this criterion, although [[Ranked Pairs]] satisfies it. [[Borda count]], [[Coombs' method]], and [[Instant-runoff voting]] fail. [[Range voting]] satisfies the criterion.
+Most [[Condorcet method]]s fail this criterion, although some, such as [[Ranked Pairs]], satisfy a related but weaker criterion known as [[local independence of irrelevant alternatives]]. [[Borda count]], [[Coombs' method]], and [[Instant-runoff voting]] fail. [[Range voting]], [[appreval voting]], and [[majority judgment]] satisfy the criterion.
 [[Arrow's impossibility theorem]] states that no voting system can satisfy universal domain, non-imposition, non-dictatorship, unanimity, and independence of irrelevant alternatives. In practice, this means that no deterministic ranked ballot system can satisfy independence of irrelevant alternatives without either having a dictator (whose ballot decides who wins no matter the other ballots), failing to elect a candidate that the whole electorate ranks first, or rendering one or more outcomes impossible no matter the ballots.