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

[Ken Taylor]

> Just became aware of this:
>
> Draft of CVD analysis about IRV vs. Condorcet Voting
> http://groups.yahoo.com/group/instantrunoff/message/1548
> (The message archives are open to everyone)
>
> I haven't gone over it in detail (yet).
>
> I would expect it to show up at http://fairvote.org soon. It would be
> great if more then one of us prepared a response to it for posting
> the moment they make something 'official'.
>

I sent a response to the author regarding his factual mistake in claiming
that Condorcet had a strategic incentive to rank a popular rival lower than
your actual preference. Here's what I wrote (criticisms welcome!):

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>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.

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).

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Ken Taylor