[EM] Simple illustration of center-squeeze effect in runoff voting

Kristofer Munsterhjelm km-elmet at broadpark.no
Wed Jan 21 02:41:24 PST 2009

Dave Ketchum wrote:
> Here it is noted that IRV has a black mark for failing to correctly 
> award W as deserving winner.  They seem not to notice that IRV's failure 
> is also describable as incorrectly discarding W as an undeserving loser.
> As to escaping two party domination, think on:
> Plurality:  If I prefer one of their candidates over the other, I must 
> vote my preference between them, and wait til next time to think of 
> voting for a minor party candidate.
> Approval:  Here I can vote for both a major and a minor, but must vote 
> as if equally desiring the barely tolerable major over the much better 
> minor.
> IRV:  See above.
> Condorcet:  Can use IRV ballots and voting, but Condorcet promises to 
> read all that I vote on them.  Further, its N*N array is a useful record 
> as to relative strength of candidates/parties.
> And "center-squeeze effect" or "Center-pull"?  Makes sense if there is 
> only one issue of interest for an election.  Makes less sense when, as 
> usually happens, there are multiple important issues with each major 
> party doing better at satisfying each voter on part of the issues.

My usual argument against Approval (in favor of something more complex) 
is this: Say there are three viable parties (if there will be only two, 
why have Approval in the first place?). You support A > B > C. If A is 
in the lead, you can approve of A alone. If A's a minor party, then you 
should approve of both A and B. But if the parties are close, then it 
may not be clear who you should approve - if A's slightly too low (and 
the important contest is A vs C), then voting only A will split the vote 
and may cause C to be elected instead of B. If A's not that low (and the 
important contest is A vs B), then voting both A and B will cancel your 
vote for A with your vote for B. It becomes more difficult the closer 
the parties are in support, and polling errors could cause further problems.

Voters shouldn't have to do this. Since we know Plurality is bad, and 
IRV is bad as well (in one sense, it has to be, so it elects the "right" 
first candidate in a multiwinner election), that leaves Condorcet - or 
something exotic like MDDA.

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