[EM] FPP vs IRV: Monotonicity. Funny example.

[MIKE OSSIPOFF]

Catchy wrote:

no, FPP is _not_ monotonic. If one ranks one's real preference
first rather than one's "lesser evil" [sic], a repugnant candidate can
win. Tell me that's not nonmonotonicity.

I reply:

First of all, in FPP we don't rank candidates. Secondly, Monotonicity
is a criterion that complies with Markus's request that the only
thing about voters that it mentiones is their votes.

Let me define Monotonicity:

If a voter changes his vote so that he votes candidate X higher
, but doesn't change his ordering of the other candidates, then
that shouldn't cause X to lose, if X would have won before the change.

By "vote X higher", I mean "vote X over someone over whom X wasn't
previously voted, or equal to someone who was previously voted over
X". Obviously, even with FPP, you vote the person who gets your vote
over everyone, and every pair of candidates has an order relation on
your ballot: All of them are equal, and one is voted over all the rest.

I've defined voting A over B or equal to B in previous postings.

FPP most definitely meets Monotonicity. Sure, it doesn't meet everything
else that we'd like it to, which is why we want to replace it. But
only with something better (not with IRV).

Mike Ossipoff


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