EM members--
This is a copy of a message that I intend to post at the Condorcet mailing
list. I have just finished requesting membership in that mailing list. I
don't know how often it takes to be approved for membership, and so I'd like
to post, to EM, three messages that I intend to post to the Condorcet
mailing list. This is the first of those three messages. They're about MDDA:
MDDA has been much discussed on EM. Its full name is: Majority Defeat
Disqualification//Approval.
Here's its definition:
1. A candidate is disqualified if another candidate is ranked over him/her
by a majorilty of the voters.
(Unless that rule would disqualify all the candidates, in which case no
one is disqualified)
2. The winner is the un-disqualified candidate who is ranked on the most
ballots.
[end of MDDA definition]
MDDA's advantages:
MDDA meets the following criteria: FBC, SFC, and SDSC.
FBC compliance means that no one ever has any incentive to vote someone else
over his/her favorite.
Neither Schulze's method nor DMC meet FBC. MDA meets FBC.
SFC compliance means that, if no one falsifies a preference (or at least it
doesn't happen on a scale sufficient to change the election result), then a
sincere-voting majority are assured that no one whom they like less than the
CW will win.
Anothe wording of SFC:
If no one falsifies a preference, and if a majority prefer X to Y and vote
sincerely, then Y shouldn't win.
Schulze's method and MDDA meet SFC. DMC doesn't meet SFC.
SDSC compliance means that if a majority of the voters prefer X to Y, then
they have a way of ensuring that Y won't win, without reversing a preference
or failing to vote their genuine preferences among all the candidates whom
they vote over other candidates.
Schulze's method and MDDA meet SDSC.
FBC is what distinguishes MDDA from Schulze's method. Voters in public
political elections have shown a pronounced tendency to vote a lesser-evil
over their favorite, to ensure that completely unacceptable candidates won'
t win. It's a common practice, and that strategy problem is known as the
lesser-of-2-evils problem.
I've observed that same favorite-burial in a presidential poll using
Schulze's method.
Some argue that Schulze's method &/or DMC won't often fail FBC. But that
isn't good enough. If voting a more winnable lesser-evil over your favorite
even slightly reduces the probability that a completely unacceptable
candidate will win, then many will do so. Only with an FBC-complying method
is it possible to absolutely and emphatically assure people that there is no
reason to rank anyone over their favorite. With BeatpathWinner or DMC you
can't give that assurance. With MDDA you can, because MDDA meets FBC.
MDDA offers FBC to the voter who needs it, and offers SFC to sincere-voting
majorities.
BeatpathWinner meets Condorcet's Criterion, and MDDA doesn't meet
Condorcet's Criterion. But I claim that Condorcet's Criterion isn't as
valuable as SFC. That's because, while Condorcet's Criterion only applies if
everyone votes sincerely, SFC merely stipulates sincere voting for the
majority to whom it makes its guarantee.
In other words, for SFC to apply, you and a majority you're in must vote
sincerely. For Condorcet's Criterion to apply, your opponents must vote
sincerely too. Which assumption is more realistic and useful?
By meeting SFC, MDDA achieves the best of what BeatpathWinner achieves.
Condorcet's Criterion is incompatible with FBC, and I suggest that FBC is
more important than Condorcet's Criterion, especially when it's possible to
get SFC in combination with FBC.
Compare the stark simpliclity of MDDA to the long definition of Schulze's
method.
Mike Ossipoff
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