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

[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>B
03: C>D
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