[EM] Draft of CVD analysis about IRV vs. Condorcet Voting

[James Green-Armytage]

Ken Taylor quoted from the CVD release:
>Suppose four candidates (A, B, C, and D) are running for an office, where
>candidates A and B are the frontrunners.
>Consider a voter whose true preferences are in order of A, B, C, D.  Under
>Condorcet, by voting insincerely this voter can
>minimize the chances that candidate B will defeat his or her preferred
>candidate A. A voter quickly realizes that the best
>strategy is to punish the strongest competitor to her favorite candidate
>by
>ranking the candidates insincerely A, C, D, B.
>Doing so may block B - and any candidate -- from becoming the Condorcet
>winner and improve candidate A's chances to
>win under the fallback rule. Worse yet, if both A and B supporters widely
>engage in such strategic voting, the winner could
>be a candidate most voters actually oppose, but didn't realize would
>benefit from their insincere rankings. With IRV, there
>rarely is an incentive to engage in strategic voting, since later rankings
>do not hurt earlier rankings.  In certain unique
>situations where there is widespread availability of detailed polling
>information, there are ways to vote strategically with
>IRV, but strategic voting is greatly limited. There is no rule of general
>applicability of the value of insincere rankings, as
>there is with Condorcet.

Ken Taylor wrote:
>This entire paragraph is completely incorrect. There is no strategic
>advantage to ranking (major candidate) B lower than your real preference
>for
>B. Try to come up with an actual vote distribution (with actual numbers)
>in
>which there is such an advantage and you'll fail. There is no change to
>the
>pairwise election between A and B -- all you're doing is giving C and D a
>better advantage against B than you really want to -- and therefore,
>you're
>not giving A any better of a chance to win against B. It's true that if
>lots
>of people also rank C and D above A, then C or D may win the condorcet
>election -- but that's not the point, here (and you address it separately
>below). The point is there is no *strategic* reason to *insincerely* do
>so.
>That is, such insincere ranking will never cause A to win over B when B
>would have won over A if you didn't rank insincerely. At best, this
>insincere ranking will create a cycle in the result -- and then the
>strategic advantage depends on the cycle-breaking method. The most popular
>cycle-breaking methods espoused by Condorcet enthusasts, such as Schwartz
>Sequential Dropping (using *magnitudes* of defeat rather than *margins*)
>can
>be shown to be immune to such strategies. Likewise, using IRV to resolve
>the
>cycle would be immune to this particular strategy.  In the interest of
>presenting the most factually correct analysis of the failings of
>condorcet,
>I would strongly suggest to remove or greatly modify this paragraph in the
>final version (unless you can find an *actual* vote distribution (with
>numbers) that shows how an insincere vote can increase the chances of your
>preferences coming about).
>----------------------------------------------
>Ken Taylor



I (James) reply:
	I'm sorry to say this, Ken, but although I am a staunch supporter of
Condorcet I think that CVD is quite correct in pointing out this
vulnerability. Indeed, this is the chief vulnerability of Condorcet's
method, and to my mind the only serious problem that it has. In the
archives from last August 

http://lists.electorama.com/pipermail/election-methods-electorama.com/2003-August/date.html

you will find a thread which I started, entitled "serious strategy problem
in Condorcet, but not in IRV?" The original message which I wrote, which
started the thread, contains two numerical examples which fit your request
above. The strategy which the CVD people are alluding to is what I now
call (borrowing Blake Cretney's term) the 'burying' strategy. I believe
that situations where opportunities for strategy are present in Condorcet
will come up relatively frequently, although by no means will they always
be exploited when they do arise. I have written a couple of proposals
about how the burying strategy might be dealt with in Condorcet using a
multiple round procedure

http://lists.electorama.com/pipermail/election-methods-electorama.com/2003-December/011393.html

but it's a tricky business. Mike Ossipoff, while recognizing the strategic
vulnerability here, does not agree with me about how serious it is. So
there's an ongoing discussion about that, and about what backup mechanisms
should be used to mitigate the problem if any. Actually I should draft
another reply in that thread in the next week or so.
	However, there is no question that the strategic possibility is there,
and so I think that your note is unfortunately quite wrong. If after
reading my August posting you no longer hold to your earlier statement, I
would suggest that you send another e-mail to CVD to concede the point. I
have to say that if you send them e-mails touting faulty arguments, you're
making Condorcet supporters look bad. I've read the CVD e-mail, and there
are effective counter-arguments to it, but that's not one of them. I wish
that Condorcet didn't have that strategy problem, but it does. Please,
let's not dismiss the CVD people as a bunch of idiots or villains. I agree
that it's an awkward situation where they control the lions share of the
public attention and donation resources toward voting methods reform and
yet are kind of lazy when it comes to voting methods theory. However, in
this case they have taken at least a little bit of time to think about it,
and many of their arguments are relatively sound, so just blindly bashing
at them is unfair and unproductive. I'm sure you meant well, and I'm sorry
to come down on you, but let's look before we leap, okay?

my best,
James