[Election-Methods] Fwd: [Election Methhods] MCA's IIB problem fixed

Kevin Venzke stepjak at yahoo.fr
Mon Jan 28 08:24:47 PST 2008


--- Chris Benham <cbenhamau at yahoo.com.au> a écrit :
> Kevin,
> In your latest post  you alluded to  MCA's failure of  Independence from 
> Irrelevant Ballots (IIB):
> >A nice thing about the majority requirement is that if it's assumed that
> >you are going to vote, there's no way that your vote can move the
> majority
> >from one candidate to another.
> >
> But but the bad thing about the "majority requirement" is that  choosing 
> between not voting and voting
> for nobody (ignoring the competitive/viable candidates) can change the 
> winner by changing the majority
> threshold.
> I suggest that MCA instead of  electing the top-ratings winner only if  
> that candidate's top-ratings score
> is greater than half the total number of valid ballots, it elects the 
> top-ratings winner (TRW)  if  the TRW's
> top-ratings score is not smaller than his/her maximum pairwise
> opposition.
> (Here I'm referring to the FBC complying version of  MCA that uses 
> 3-slot ratings ballots, but this
> mechanism could equally be used for the version you were discussing that 
> uses hybrid FPP-approval
> ballots, and also to Bucklin.)
> Doesn't this fix MCA's  IIB problem at no cost (except a bit more 
> complexity)?

Well, if you're considering using MCA then you probably care about
complexity. I also don't find the FBC-satisfying version of MCA to be a
significant improvement over Approval.

In terms of criteria, at first glance it seems like this has a good chance
of preserving FBC since pairwise opposition is friendly to it. I'm not
totally sure.

As far as my strategy simulation is concerned, this rule change raises the
question of how voters should evaluate the possibility that they elevate a
candidate to the top spot on first preferences only to see him lose due to
pairwise opposition.

Kevin Venzke

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