[EM] Citation for immunity to strategic voting?

Andrew Myers andru at cs.cornell.edu
Mon Sep 12 21:19:55 PDT 2005

On Sun, Sep 11, 2005 at 04:47:19PM -0400, Andrew Myers wrote:
> On Mon, Sep 05, 2005 at 05:55:01PM -0400, Stephane Rouillon wrote:
> > Actually as many people will tell you,
> > this claim is wrong.
> > 
> > I see that Rob already gave you a counter example.
> > 
> > Maybe you would like to know that using winning vote as
> > criteria to make pairwise comparison instead of margins
> > can make your claim true for strong Condorcet winners
> > (ones which have a more than 50% majority against every
> > other candidate).

Actually even this weaker claim (as I understand it) is wrong. Consider the
following election with 100 voters:

23 A>B>C
25 A>C>B
3  B>A>C
26 B>C>A
3  C>A>B
20 C>B>A

Therefore we have A preferred to B 51-49, A preferred to C 51-49, and B
preferred to C 52-48. So A is a strong Condorcet winner.  But consider what
happens when the 3 B>A>C voters decide to bury A, changing their ballots
to B>C>A. Then a cycle results:

A vs. B: 51-49
B vs. C: 52-48
C vs. A: 52-48

According to all wv methods, we drop the weaker A vs. B preference, and B wins.

-- Andrew

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