[EM] Method definitions
Markus Schulze
schulze at sol.physik.tu-berlin.de
Tue Feb 29 04:52:23 PST 2000
Dear participants,
on page LXVIII of his "Essai sur l'application de l'analyse a la
probabilite des decisions rendues a la pluralite des voix"
(Imprimerie Royale, Paris, 1785), Condorcet writes due to my own
translation:
> From the considerations, we have just made, we get the general
> rule, that in all those situations, in which we have to choose,
> we have to take successively all those propositions that have
> a plurality, beginning with those that have the largest,
> & to pronounce the result, that is created by those first
> propositions, as soon as they create one, without considering
> the following less probable propositions.
On page 126, he writes due to my own translation:
> Create an opinion of those n*(n-1)/2 propositions, which win
> most of the votes. If this opinion is one of the n*(n-1)*...*2
> possible, then consider as elected that subject, with which this
> opinion agrees with its preference. If this opinion is one of the
> (2^(n*(n-1)/2))-n*(n-1)*...*2 impossible opinions, then eliminate
> of this impossible opinion successively those propositions, that
> have a smaller plurality, & accept the resulting opinion of the
> remaining propositions.
******
I interpret the first quotation as follows: "Minimize the maximum
pairwise defeat that has to be ignored to get a complete ranking
of the candidates."
I interpret the second quotation as follows: "Maximize the minimum
pairwise defeat that has to be used to get a complete ranking of the
candidates."
Therefore I think that every election method, that minimizes the
maximum pairwise defeat that has to be ignored and maximizes the
minimum pairwise defeat that has to be used to get a complete
ranking of the candidates, is a possible interpretation of
Condorcet's words. Therefore I think that the Tideman method,
the Schulze method and many other election methods are possible
interpretations of Condorcet's words.
******
Example (3 Feb 2000):
26 voters vote C > A > B > D.
20 voters vote B > D > A > C.
18 voters vote A > D > C > B.
14 voters vote C > B > A > D.
08 voters vote B > D > C > A.
07 voters vote D > A > C > B.
07 voters vote B > D > A = C.
Then the matrix of pairwise defeats looks as follows:
A:B=51:49
A:C=45:48
A:D=58:42
B:C=35:65
B:D=75:25
C:D=40:60
1. The "strongest cycle" is C > B > D > C. The weakest pairwise
defeat of this directed cycle is C:D=40:60. Therefore the
ranking of the candidates should be compatible to all
pairwise defeats that are stronger than 40:60. That means
that the final ranking must include C > B and B > D.
2. If votes-against was used then A:C would be the weakest
pairwise defeat. Therefore he would ignore A:C=45:48. Then he
would ignore A:B=51:49. Then he would take A:D=58:42 into
consideration because otherwise it wouldn't be possible anymore
to get a complete ranking of the candidates. That means that
the final ranking must include A > D.
Therefore I conclude that every election method that is
compatible to Condorcet's words must result in a ranking that
includes C > B, B > D and A > D.
Markus Schulze
(this time without any virus)
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