[EM] élection de trois élection de trois

Kevin Venzke stepjak at yahoo.fr
Thu Feb 23 17:15:38 PST 2012


De : Kristofer Munsterhjelm <km_elmet at lavabit.com>
>As a consequence, among ranked methods, some really bad methods (like Plurality) 
>gets it wrong when there are two candidates plus no-hopes; some slightly better 
>methods (like IRV, and perhaps I'd also put DAC/DSC here since it uses the same 
>logic) can identify and remove the no-hopes but then gives bad results when the 
>going gets tough; while yet other methods (such as Condorcet) use more consistent
>logic and, though not perfect, handle three-way (and n>3 n-way) races much better.

I guess I might measure this as the need to compromise or compress, since this is
what you probably do when the method won't handle the third candidate well. One 
figure I like to compute is the % of voters compromising plus half the % that 
compress. If I do that I get this for 1D scenarios:
17.1% FPP
16.3% Approval
9.2% DSC
7.9% TACC (the worst-scoring Condorcet method)
6.5% IRV
3.9% DAC
0.1% AWP explicit (the best-scoring Condorcet method)
I can also look at the % of voters who feel the race was spoiled by one of the
non-winning candidates (in a 3-candidate race):
12.0% FPP
2.2% Approval
2.1% DSC
0.98% Stensholt's BPW (the worst-scoring Condorcet method)
0.91% IRV
0.59% DAC
0.08% AWP explicit (the best-scoring Condorcet method)
(There are some changes if I don't assume there is issue space. In particular,
by the latter measure IRV and FPP see drastic improvement in relation to others.)
>Rated method supporters, like Warren, would likely say that the rated methods are 
>even better since they can pass IIA and so can scale to any number of candidates. 
>They do pass IIA, but in exchange people have to be able to say how much they like 
>a candidate rather than just better/worse-than, and it doesn't get around Gibbard-
The voters also have to be willing to give a candidate the same score no matter
which other candidates are in the race. So 10/10 can only go to the best possible
candidates, 0/0 to the worst. If a 10/10 candidate enters the race, you can't 
demote a candidate to make room for the new one. If voters can't behave like this,
then the IIA compliance doesn't amount to much.

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