From Wikipedia, the free encyclopedia.
In voting systems, 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.
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.
All Condorcet methods fail the former criterion, but some satisfy the latter.
None of the Borda count, Coombs' method or Instant-runoff voting meet either criterion.
An anecdote which illustrates a violation of this property has been attributed to Sidney Morgenbesser:
- After finishing dinner, Sidney Morgenbesser decides to order dessert. The waitress tells him he has two choices: apple pie and blueberry pie. Sidney orders the apple pie. After a few minutes the waitress returns and says that they also have cherry pie at which point Morgenbesser says "In that case I'll have the blueberry pie."
