[EM] Manipulability

[Forest Simmons]

On Sat, 31 May 2003, [iso-8859-1] Kevin Venzke wrote:

>  --- Alex Small <asmall at physics.ucsb.edu> a écrit :
> > MCA is also more likely to elect A in this scenario.  The A>B>C faction
> > can safely rate A preferred and B acceptable, unless they believe a
> > majority of the electorate is rating both B and C as preferred.  So while
> > MCA is not completely immune to manipulation of polls (is anything?), it's
> > certainly more robust than some methods.
>
> Looks that way.  A supporters who think A could have a majority might even
> disapprove B.  However, if they think C might have a majority of "preferred"
> ratings, they might bump B up to "preferred" along with A.  This scenario
> seems more likely than what you envisioned.
>

...

>
> I want to consider the case with three-rank Conditional Approval (of interest
> only to me, perhaps):
>
> Sincere:
> 6000 A>>B>C
> 3000 C>B>>A
> 1000 B>C>>A
>
> False poll:
> 35% A>B>C
> 40% C>B>A
> 25% B>C>A
>
> Believing these poll results, the A supporters have no reason not to
> vote A|B|C.  Why?  They only end up supporting B if C takes the lead.  They
> don't risk keeping A from beating B in this way, because they don't believe
> any of the B/C voters are willing to compromise on A.  So the thought is,
> if C leads, A is no longer viable, so why not support B.
>
> The strategy of B and C voters is more complicated, but it doesn't make
> a difference in the outcome.  The results would be:
> 6000 A|B|C
> 3000 C|B|A or C||BA
> 1000 B|C|A or B||CA
>
> A is the initial leader in any case.  In response, B and C can't get
> any more than 4000 votes total.  Thus A remains the leader and wins.
>
>
> Question: What advantage does MCA have over three-rank Condorcet?
>

Good Question!

I think it's easier to explain to the lay voter: "If nobody's favorite
gets more than fifty percent of the votes, then the most acceptable
candidate wins."

We need to compare the two methods on more examples to clarify the other
advantages and disadvantages.

Forest

P.S.

The Strong FBC method holds up well on this example because the B and C
supporters have no incentive to approve any pair other than {B,C} and the
A supporters have no incentive to approve any pair other than {A,B}. The
finalist pair then turns out to be {A,B} and A wins.