[EM] IA/MPO

[Kevin Venzke]

Hi Forest,

> De : Forest Simmons <fsimmons at pcc.edu>
>À : Kevin Venzke <stepjak at yahoo.fr> 
>Cc : em <election-methods at electorama.com> 
>Envoyé le : Mercredi 9 octobre 2013 19h51
>Objet : Re: [EM] IA/MPO
>
>Kevin,
>
>thanks for working on the property compliances.
>
>I agree that this method does satisfy the FBC, is monotone, and is at least marginally clone independent, like Score and ratings based Bucklin and MMPO.
>
>I am not as expert as you in the various defense criteria.
>
>My main focus so far is that the method seems to remedy some of the problems of Approval and some of the problems of MMPO.  


Unfortunately, I realized that an SFC problem is possibly egregious:

51 A>B
49 C>B

B would win easily, contrary to SFC (which disallows both B and C). But more alarmingly it's a majority favorite problem.

>Approval has a problem with this (true preferences) scenario:

>
>30 A
>3 A>C
>15 C>A
>4 C
>15 C>B
>3 B>C
>30 C
>
>Of course, (under Approval voting) the two 15 member factions should, and would bullet C,  if they were sure of the numbers, but it is more likely that due to disinformation from the A and B parties (and other sources of uncertainty) they would not truncate their second preferences, so A and B would be tied for most approval.

Yes, Approval can't easily adjust on election day if polls were inaccurate or there is a sudden change of sentiment. It tends to settle on two frontrunners, and when it doesn't, the outcome is fairly arbitrary (which isn't better in my opinion).


>
>However, IA/MPO robustly elects C.
>
>Our friend MMPO has a problem with
>
>19 A>B>C
>18 B>C>A
>18 C>A>B
>15 D>A>B
>15 D>B>C
>
15 D>C>A
>
>electing the Condorcet Loser D, (unless some preferences are strategically collapsed).  But IA/MPO elects the "right winner" A, with no need to collapse preferences among members of the ABC clone set..
>
>Can you think of any other examples where one or the other of IA or MMPO is by itself inadequate?  Does IA/MPO always improve the outcome in such cases?
Well, "implicit approval" as a method on its own wouldn't be proposable, I don't think, due to the majority favorite issue.

MMPO isn't fantastic at deterring burial, so I was hoping the IA might address that as in implicit Condorcet//Approval (though plain C//A is not great wrt FBC).


Kevin Venzke