[EM] IRV and Brown vs. Smallwood

[Juho Laatu]

--- On Thu, 9/4/09, Kristofer Munsterhjelm <km-elmet at broadpark.no> wrote:

> Juho Laatu wrote:
> 
> > Actually it may be a quite good strategy in
> > IRV not to rank those favourite candidates
> > that do not have a chance but to rank only
> > those candidates that have a chance. This
> > increases the probability that one's most
> > favoured candidates with chances of winning
> > the election are not eliminated too early
> > (assuming that they might win if they could
> > stay in the race until the end).
> 
> That's also the strategy in Plurality - don't vote for
> those candidates that don't have a chance. But if everybody
> thinks like this, you end up with the "lesser of n evils".

Yes, it is true that if people do not rank
the lesser candidates they will never grow
and become major candidates. It may be more
important for many to try to influence the
future elections than to try to eliminate
some small risks in this election. And of
course in many cases one can vote also for
the lesser candidates without problems. The
described strategy is just a safe bet that
eliminates risks in these elections.

(Psychological factors are an important
topic that should be covered too.)

> One of the points of ranked voting is that you don't have to
> do that - you can vote X > Y > Z so you say "I like X,
> but if I can't have X, I'd have Y before Z".

It seems that in IRV it is the safest
strategy not to rank the weak candidates
(if one only aims at winning this election
in question), but not a necessary strategy
for all situations to guarantee an optimal
vote.

>    If this ability
> is compromised by that voting for unpopular candidates
> dilutes the vote so much one should rather not, then why
> have ranked voting in the first place?

Words "so much" are important. Polls are
inaccurate, people do believe in the
chances of their favourites, there will be
changes in support, there is a need to
show support to the "so far unpopular"
candidates, and the risks involved in this
strategy may be small. As a result I don't
think people should and people will apply
this strategy generally in IRV elections.

There are however cases where the risks
are very real. The original example was
one. Here is another with moderate and
radical Democrats and Republicans.

Approximate support:
25: Dr>Dm>Rm>Rr
20: Dm>Dr>Rm>Rr
05: Dm>Rm>Dr>Rr
05: Rm>Dm>Rr>Dr
05: Rm>Rr>Dm>Dr
25: Rr>Rm>Dm>Dr

In this example all four candidates have
the risk of being eliminated early. If Dm
or Rm will be eliminated first then the
other party is likely to win. It makes
sense to the Dr and Rr supporters not to
rank their favourite first (although they
are about as popular within the party as
the other moderate candidate).

(From this point of view Condorcet methods
allow the voters to use more sincere
rankings than IRV.)

Juho