[EM] Arrow's Theorem.

[Markus Schulze]

Dear Forest,

it is true that FPP isn't a preferential method according to
the everyday use of the term "preferential." However, FPP is a
preferential method (even when lone mark ballots are being used)
according to Arrow's use of the term "preferential" since FPP
can be used to every possible set of orders of merit and since
the FPP winner doesn't depend on information that is not included
in the set of orders of merit.

> It is interesting that FPP plurality fails the IIAC when the method
> makes use of ranked preference ballots, but passes the IIAC when
> lone mark ballots are used, since second place preferences cannot be
> inferred from the lone mark ballots ... those voters who voted for
> the candidate that drops out just end up without a vote in this
> latter case, and the original plurality winner still has the greatest
> number of votes.

To be a "preferential" method it is necessary that the winner doesn't
depend on more than the set of orders of merit. It is not necessary
that the winner doesn't depend on less than the set of orders of merit.
The fact that every election method collects only that information that
it needs to calculate its winner is obvious but irrelevant.

Markus Schulze