[EM] A more briefly-defined method with the best mix of properties

[MIKE OSSIPOFF]

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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