[EM] IRV in action

[Markus Schulze]

Dear James,

I wrote (6 April 2003):
> I guess that for every reasonable method there are situations where
> "bullet voting" is a useful strategy. I guess that when IRV is being
> used and there are three potential winners, then each party will
> ask their supporters to bullet vote.

You wrote (7 April 2003):
> Why would they do that?  How could it help secure the election of
> their candidate?

Let's say that there are 3 potential winners: X, Y, and Z.
Then candidate X will ask his supporters to bullet vote hoping
that those voters who have candidate X as their sincere second
preference will give their first preference to candidate X
since they fear that their sincere first preference cannot
win since their sincere preference won't get additional
votes when candidate X is eliminated since the supporters
of candidate X bullet vote.

In federal elections in Australia, bullet voting is not allowed.
In the USA, the mayors were elected by FPTP even where the city
councils were elected by PR-STV. Therefore, there are too few
real IRV elections to say whether bullet voting is a problem
under IRV.

However, when PR-STV is being used then it sometimes happens that
there are three potential winners for the last seat. In this
situation, bullet voting is a frequently used strategy. Typical
examples are the black community in the USA, the Communist Party
in New York City, and Sinn Fein in the Irish Republic. Mart Bax
("Harpstrings and Confessions," University of Amsterdam, 1973)
also mentions a case where in Dail elections a candidate asks his
supporters to bullet vote to keep a party rival from winning; but
I believe that this case is rather atypical.

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 bullett vote.
   Then this example looks as follows:

   40 voters vote X > Y > Z.
   35 voters vote Y > Z > X.
   25 voters vote Z.

   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.

   Now candidate Z is the IRV winner.

Markus Schulze