[EM] RE : Re: Election methods in student government...

[Chris Benham]

Chris Benham wrote:

> 38: A
> 19: B>C>D
> 17: B>D>C
> 10: C>B
> 03: C>D
> 10: D>C
> 03: D>B 

My example here of  DSC failing both  DMT and Condorcet Loser works, but 
not quite what I meant to type:

38: A
19: B>C>D
17: B>D>C
10: C>D
03: C>B
10: D>C
03: D>B

(I've "corrected" it below as well).

Chris Benham


>
> Tim Hull wrote:
>
>> DSC uses a somewhat interesting method - it effectively goes and 
>> excludes the groups of candidates that the most people prefer a 
>> "solid coalition" to until it finds a winner.  However, what I am 
>> wondering is - what are the primary flaws of these two methods 
>> (especially as compared with IRV, of which I know quite a bit about 
>> the flaws)?
>
>
> DSC fails several important (in my book) criteria that are met by IRV.
>
> DSC fails "Dominant Mutual Third", which says that if  there is a set 
> of candidates X that all pairwise beat all the outside-the-set 
> candidates and they are "solidly supported"
> (ranked above all the outside-the-set candidates) on more than a third 
> of the ballots, then the winner must come from X.
>
> 49: A
> 48: B
> 03: C>B
>
> Here the DMT set is {B}, but DSC elects A.
>
> (If the B voters switch to B>C then B wins, a failure of  
> "Later-no-Help".)
>
> DSC fails "Condorcet Loser", which says that a candidate that is 
> pairwise beaten by every other candidate mustn't win
>
> 38: A
> 19: B>C>D
> 17: B>D>C
> 10: C>D
> 03: C>B
> 10: D>C
> 03: D>B
>
> DSC fails Condorcet Loser by electing A.
>
> This is also a failure of  Dominant Mutual Third (DMT), by not 
> electing B.
>
> IRV is invulnerable to the Burying strategy.
>
> 49: A
> 48: B>A
> 03: C>B
>
> DSC elects A, but if the B>A voters change to B>C then their  burial 
> strategy against A succeeds and B wins.
>
> DSC has a "random-fill" incentive and so fails what I call "No 
> Zero-Information Strategy". In the 0-info. case the DSC voter
> gets a better expectation by strictly ranking all the candidates, if 
> necessary at random; whereas the IRV voter does best to
> rank sincerely.
>
> I think of DSC as just FPP that has been minimally improved to meet  
> Clone-Winner and  Majority for Solid Coalitions.
>
> Chris Benham
>
>
>
>