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

Markus Schulze markus.schulze at alumni.tu-berlin.de
Sat Feb 2 03:10:43 PST 2002

```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.

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

```