[EM] Sum-up for Ranked Pairs

Blake Cretney bcretney at postmark.net
Thu Apr 26 09:10:45 PDT 2001

I'm going to give a summary of my arguments for why you should vote
for Ranked Pairs(m) in first place, or at least pretty high up on your

I haven't really debated Approval much.  I think people want to be
able to specify more of their preferences than an Approval vote
allows.  However, if you do favour Approval, I ask that you consider
Ranked Pairs as a second choice.

The margins vs. winning-votes issue has already been discussed quite a
bit, and it looks pretty clear that margins will win anyway, so I
won't address that further either.  I will just state that you should
remember to vote for Ranked Pairs(m) if you want Tideman's original
margins based method.

Judging by the email, the Condorcet race is largely Ranked Pairs vs.
the Schulze family of methods.  Here, I don't want to overstate the
case.  Assuming margins, these methods are very similar.  It seems
unlikely that in real elections they will differ in their answers very
often, if at all.  But that's really an argument for Ranked Pairs.  It
is older, and has already been published, so the onus should be on
other methods to show their superiority.  Also, it is simpler and more
straight-forward.  The only way I can convince you of that, is to look
at the definitions of each method, and judge for yourself.  Remember,
though, that words like "Schwartz" and cycle would have to be
explained to the general public.

It is true that Ranked Pairs is more difficult to program than
Schulze.  However, it doesn't make sense to choose a method for the
benefit of programmers.

The main argument that has been brought in favour of Schulze is that
in a simulation, it achieved a slightly better approximation of
ratings than did Ranked Pairs, both falling behind Borda.  It seems to
me that there are problems with choosing Schulze on this basis.  There
is no reason to believe that Schulze ratings are optimal, even
assuming important criteria.  It puts us in the position that we may
be advocating Schulze, only to be later faced with another method with
an imperceptibly higher ratings score, and even greater complexity. 
Would we then abandon Schulze?  Also, such a small difference might
very well be susceptible to changes in the model of voter preferences
used.  In fact, although I expect it will hold out, there have not yet
been any arguments showing statistical (let alone practical)
significance for the results.

Read my signature for a brief definition of Ranked Pairs.  Note that
although this definition doesn't suggest an efficient procedure to
carry out the method (though one exists), it does define the method
and suggest a justification, which is really all you need to advocate
it.  Some terms may require clarification, but they have the meaning
one would expect.

Blake Cretney   http://www.fortunecity.com/meltingpot/harrow/124/path

Ranked Pairs gives the ranking of the options that always reflects 
the majority preference between any two options, except in order to
reflect majority preferences with greater margins. 
(B. T. Zavist & T. Tideman, "Complete independence  of clones in the 
ranked pairs rule", Social choice and welfare, vol 6, 167-173, 1989)

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