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Line 1: {{wikipedia\|Pareto efficiency}} ~~The~~'''Pareto efficiency''' (frequently referred to as the "'''Pareto criterion'''" or "'''Unanimity criterion'''" in the [[election method]] context) is a basic criterion for evaluating [[voting system\|voting systems]]. It can be defined in this way: {{definition\|If every voter ~~ranks~~prefers ~~candidate~~alternative ~~''A''~~X ~~above~~over ~~candidate~~alternative ~~''B''~~Y, then ~~''B''~~the ~~must~~election ~~not~~method beprefers ~~elected.~~X over Y}} This criterion is important in the context of [[Arrow's impossibility theorem]], since it was one of Arrow's criteria in the theorem. Virtually every devised election method satisfies this criterion. An example of a method which would fail it would be ''Random Candidate'', where some candidate is elected at random, regardless of the submitted votes. A second, stronger variation of the criterion (meaning it implies the first variation of the criterion as well) is "if at least one voter prefers X over Y, and no voters prefer Y over X, then the system prefers X over Y." '''Independence of Pareto-dominated alternatives (IPDA)''' 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 (using the second version of the criterion). The Pareto criterion is the single-winner case of Hare-[[PSC]]. [[Category:Voting system criteria]]