Terry Bouricius wrote:
Kathy,
On the monotonicity criterion and IRV. IRV does indeed fail the most
common definition of the monotonicity criterion, as do two-round runoffs
and all other methods (I believe) that satisfy the later-no-harm
criterion. It is a trade off ... Which is more important in the real
world, monotonicity or later-no-harm. Different experts have different
opinions on that.
Just a note in passing: if you judge electoral methods just by whether
they fulfill monotonicity or LNHarm, then you should pick Descending
Solid Coalitions, which passes both.
In practice, though, DSC can give weird results; Chris Benham showed one
in particular. What the existence of DSC does show is that it's possible
to make a method that passes both monotonicity and LNHarm.
Plurality also technically passes both of the LNH criteria, as well as
monotonicity. When I said that was impossible in an earlier post, I
implicitly assumed one would want mutual majority, which Plurality
fails. To pass mutual majority, if there's a group of candidates that a
majority prefers to those not in the group, then the system should
always pick a candidate within that group.
----
Election-Methods mailing list - see http://electorama.com/em for list info
