[EM] IRV in action

[Dave Ketchum]

On Tue, 08 Apr 2003 12:08:31 +0200 Markus Schulze wrote:

> Dear Dave,
> 
> I wrote (7 April 2003):
> 
>>Example:
>>
>>   40 voters vote X > Y > Z.
>>   35 voters vote Y > Z > X.
>>   25 voters vote Z > Y > X.
>>
>>   Candidate Y is the IRV winner.
>>
>>   Suppose that candidate Z asks his supporters to bullet vote.
>>   Then this example looks as follows:
>>
>>   40 voters vote X > Y > Z.
>>   35 voters vote Y > Z > X.
>>   25 voters vote Z.


How did we get here?  We started with Z voters liking Y better than X. 
Why would they destroy that via bullet voting?

>>
>>   Now candidate X is the IRV winner. Now candidate Z can hope
>>   that some of the supporters of candidate Y will give their
>>   first preference to candidate Z to keep candidate X from
>>   winning. Then this example looks as follows:
>>
>>   40 voters vote X > Y > Z.
>>   35 voters vote Z > Y > X.
>>   25 voters vote Z.


How did this ever happen?  After the Z voters make sure Y could not win, 
what kind of arm twisting is ever going to get Y voters to help Z win???

>>
>>   Now candidate Z is the IRV winner.
>>
> 
> You wrote (7 April 2003):
> 
>>Given the candidate's positions being equidistant:
>>      IRV - I see not how bullet voting matters, for nothing else is seen
>>until/unless your candidate loses.
>>
>>Given two candidates being siblings, far from the third:
>>      IRV - voters for a sibling candidate almost certainly want to give
>>the other sibling second preference, to decrease the chance of the third
>>candidate getting elected.  I would expect these two candidates to promote
>>this voting.  I do not see the third candidate caring, as I said above.
>>
> 
> Suppose that there are three potential winners: X, Y, and Z.
> Suppose that Y pairwise beats X when every voter votes sincerely.
> Suppose that Z pairwise beats X when every voter votes sincerely.
> Suppose that Y cannot pairwise beat X without the votes of the Z supporters.
> 
> Then it makes sense for the Z supporters to bullet vote so that the
> Y supporters see that Y cannot win. At least those Y supporters who
> have Z as their sincere second preference will then give their first
> preference to Z so that Z wins.


One detail is that, in a normal public election, everyone casts their vote 
before they learn what others have done (if you get 6 promises to vote for 
Z, and Z gets a total of 5 votes, you know that at least one of the 6 did 
not vote for Z).  Thus you do not learn what others are doing before you 
and they have all voted.  In turn, you do not learn whether it is possible 
for Y to win until too late for your evil schemes.

Looking back to what I wrote:
      If X and Y are siblings, we will see X>Y and Y>X, and Zs can be 
tolerated doing bullet votes, though Z>X and Z>Y would be welcome.
      In the equidistant environment, with equal strength candidates, I do 
not see anyone cooperating with anyone who goes in for bullet voting.

     In the equidistant environment, with Z being weak, I still do 

not see anyone cooperating with anyone who goes in for bullet voting.
> 
> Markus Schulze

-- 
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   Dave Ketchum    108 Halstead Ave, Owego, NY  13827-1708    607-687-5026
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