[Election-Methods] Unnecessary voting method? (was: How to use rankings below approval cut-off)
Steve Eppley
SEppley at alumni.caltech.edu
Sun Dec 16 08:16:48 PST 2007
Hi,
Assuming I'm correctly understanding a voting method Stéphane Rouillon
used in a recent message (excerpted below), which he called "Repetitive
Condorcet (Ranked Pairs(Winning Votes)) elimination," it is
unnecessarily complicated because it chooses the same winner as Ranked
Pairs(Winning Votes), which of course is simpler.
Ranked Pairs(Winning Votes), also known as MAM, satisfies H Peyton
Young's criterion Local Independence of Irrelevant Alternatives (LIIA).
One implication of LIIA is that elimination of the last-ranked
candidate(s) does not change the ranking of the remaining candidates.
By the way, a different criterion has been masquerading as LIIA in
Wikipedia. Peyton Young defined the real LIIA in his 1994 book Equity
In Theory And Practice (if not earlier).
--Steve
--------------------------------------
Stéphane Rouillon wrote:
-snip-
> Let's try a counter-example:
>
> 3 candidates A, B, C and 100 voters.
> Ballots:
> 35: A > B > C
> 33: B > C > A
> 32: C > A > B
>
> Repetitive Condorcet (Ranked Pairs(winning votes) ) elimination would
> produce
>
> at round 1:
> 68: B > C
> 67: A > B
> Thus ranking A > B > C
> C is eliminated.
>
> at round 2:
> 67: A > B is the ranking
> B is eliminated
>
> at round 3:
> A wins.
-snip-
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