[EM] Consensus?: IRV vs. Primary w/Runoff

[Markus Schulze]

Dear Forest,

you wrote (1 Feb 2002):
> Markus wrote (1 Feb 2002):
> > Forest wrote (1 Feb 2002):
> > > It seems to me that Primary with Runoff is virtually independent from
> > > clones, too, because it would take at least a two thirds majority to
> > > get two clones as finalists, unless the voters are asleep.
> > 
> > When primary with runoff is used then a party doesn't need a two thirds
> > majority to manipulate the result of the elections by running clones.
> > It is only necessary that the clones take so many votes away from the CW
> > that he falls behind the candidate with the third largest number of first
> > preferences.
>
> How can they do this if they have less than two thirds of voters on their
> side? If more than one third of the voters don't want two clone candidates
> to survive to the finals, they can agree to vote for the CW or some other
> candidate that they prefer and prevent one of the clones from surviving to
> the finals.

Let's say that candidate A gets 35%, candidate B gets 30%, candidate C
gets 25% and candidate D gets 10% of the first preferences. Let's say that
candidate B is the Condorcet winner. Let's say that candidate A would win
the runoff against candidate C. Then to make candidate A win the elections,
it is sufficient that clones of candidate B take so many first preferences
away from candidate B that he gets less than 25% of the first preferences.

******

You wrote (1 Feb 2002):
> Markus wrote (1 Feb 2002):
> > Forest wrote (1 Feb 2002):
> > > It (the instant version of Primary with Runoff) is summable, has
> > > simpler strategy, is less likely to eliminate the sincere CW, and
> > > (I believe) is less manipulable.
> >
> > It is questionable whether summability is desirable. E.g. Nurmi
> > and Bartholdi consider summable methods to be more manipulable.
>
> They have a strange conception of manipulability.

Due to Nurmi and Bartholdi, the more information you need about the
opinions of the voters resp. the more accurate this information must
be to be able to calculate a strategy, the less vulnerable to strategies
the used election method is. To my opinion, this argument by Nurmi and
Bartholdi is plausible.

******

You wrote (1 Feb 2002):
> Markus wrote (1 Feb 2002):
> > And your argument that primary with runoff "has simpler strategy"
> > is rather an argument against primary with runoff.
>
> In what way?

When an election method has "simpler strategy" then, of course, this
method is more vulnerable to strategies.

Markus Schulze