Independence of Smith-dominated alternatives
Electoral system criterion / From Wikipedia, the free encyclopedia
Independence of Smith-dominated alternatives (ISDA, also known as Smith-IIA) is a voting system criterion which says that the winner of an election should not be affected by candidates who are not in the Smith set.[1]
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Say we classify all candidates in an election into two categories, Frontrunners and non-Frontrunners, where every candidate in the group of Frontrunners defeats every candidate in the group of non-Frontrunners. Then, independence of Smith-dominated alternatives says it is always possible to eliminate all candidates in the group of non-Frontrunners without changing the outcome of the election.
Another way of defining ISDA is to say that adding a new candidate should not change the winner of an election, unless that new candidate beats the original winner, either directly or indirectly (by beating a candidate who beats a candidate who... who beats the winner).