# [EM] IRV in action

Dave Ketchum davek at clarityconnect.com
Tue Apr 8 21:15:04 PDT 2003

```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
>>
>>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

--
davek at clarityconnect.com    http://www.clarityconnect.com/webpages3/davek
Dave Ketchum    108 Halstead Ave, Owego, NY  13827-1708    607-687-5026
Do to no one what you would not want done to you.
If you want peace, work for justice.

```