The ITA Criterion

[Mike Ossipoff]

As Steve said, this should be counted as a criterion, so I'm going
to follow up on that:

The Invulnerability to Twin Alternatives Criterion (ITA):

Definition: Alternative T is a "twin" to alternative X if everyone
votes T over every alternative over which they vote X, and if
everyone votes everything over T that they vote over X.

Definition of ITA Criterion:

A method meets ITA if & only if the addition of a twin can't
change the election result (where the election of an alternative
& the election of a twin of that alternative aren't counted
as different results).


***

Plain Condorcet & Smith//Condorcet meet ITA, but every Copeland
version, including Regular-Champion, fails ITA. In fact, Copeland,
in all its versions, along with Plurality, is one of the few
methods that fail ITA.

***

Mike





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