Adam Tarr (last name?): It is not quite so obvious that the Instant 2-stage plurality/pairwise runoff, or "instant two stage runoff" (ITTR), is better that IRV, overall. I am still waiting for the mathematical proof, and quantification of the different properties. If that is ever possible, it should be possible to quantify and prove the differences between these particular 2 voting methods, as they are as similar as can be.
I must point out that the IRV is more likely to choose the Condorcet Winner when that candidate is not among the top 2 plurality vote getters. In such a case, the CW's chances are ZERO with the ITTR, but they are much better than that with the IRV. On the other hand, if the Condorcet Winner is among the top 2 plurality vote getters, the CW's chances are much better than zero with the IRV. So, you see, it is not quite so obvious as it seemed at first blush that a Condorcet Winner's chances are better with the ITTR than with IRV. Furthermore, remember that the two methods are identical with 3 candidates. It may be that the more candidates you have the less likely it is that the CW will be among the top 2 plurality vote getters. Can somebody prove one of these 2 methods is better than the other in any of these respects? I have already proven (informally and in my own mind, at least) that the IRV is less manipulable than the traditional (non-instant) 2-stage plurality/pairwise runoff with 4 or more candidates, but I don't know how the instant version of that method compares to IRV in that respect. I suspect that IRV is less manipulable than ITTR, too, simply because it potentially involves more stages, and is, thus, more complicated and harder to predict. Anyway, the comparison of these 2 instant runoff methods seems to merit some attention, don't you think? Formal proofs and illustative examples can be very useful and convincing. SB PS: Can we leave the Approval Vote method out of this, for now? > Date: Tue, 12 Mar 2002 11:31:52 -0500 (EST) > From: [EMAIL PROTECTED] > To: [EMAIL PROTECTED] > Subject: Re: [EM] Instant 2-stage plurality/pairwise runoff > > Steve Wrote, > > > I am interested in the fact that the traditional 2 stage plurality > > runoff will always choose a Condorcet Candidate, when one exists, > > if that candidate is among the top 2 plurality candidates in the > > first round. This suggests that we should consider an instant 2-stage > > runoff procedure where preference ballots are used, and the top two > > plurality candidates go into an instant pairwise vote. > > It does not suggest this. The key problem is that one caveat you mention, > "if > that candidate is among the top 2 plurality candidates in the first round." > We > have no guarantee that this will be the case, and indeed it will not be the > case > in many scenarios. In particular, the classic (on this list anyway) "left > middle/right" scenario can cause middle, the obvious Condorcet winner, to > finish > last in the first round of plurality. Demorep likes to use the stark Stalin > Wahington/Hitler example to illustrate this. > > As an interesting side note, the nation of Sri Lanka (just south of India) > recently switched over to such an instant two stage runoff (we called it ITTR > in > a thread a few months ago, for instant top two runoff). Thus far, their ITTR > > elections have produced the same results that a plurality election would > have. > It should be interesting to watch. ITTR is better than IRV, although both > are > pretty bad. > > -Adam __________________________________________________ Do You Yahoo!? Yahoo! Sports - live college hoops coverage http://sports.yahoo.com/
