> 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!): ---------------------------------------------- >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). ---------------------------------------------- Ken Taylor ---- Election-methods mailing list - see http://electorama.com/em for list info