There's a website with an article that lists some strategies that are possible with various single-winner voting systems. It says that Condorcet is subject to "compromise strategy", meaning that voters have an incentive to insincerely up-rate a compromise. If we ask that no one ever have strategic incentive to vote a less-liked candidate equal to or over a more-like one (meaning that no one can ever conceivably gain by doing that), then no method meets that criterion. Say we ask that no one ever have strategic incentive to vote a less-liked candidate equal to or over _their favorite_. Still no can do. No method meets that criterion. Then what about asking that no one ever have strategic incentive to vote a less-liked candidate over one's favorite? That we can do. Approval meets that criterion. No other method does. But let's loosen it up a bit, and, instead of talking about what incentive some individual voter could have, let's talk about what a majority has to do to get something that it all agrees on. A majority has the power to elect anyone they want to, or to prevent the election of anyone they want to (At least unless the method is Borda). So it's of interest what they have to do in order to achieve such a result. With almost all of the methods that have been discussed here a majority can elect anyone they all like best, without insincere strategy. So let's ask that a majority have a way to ensure that someone won't win, without any member of that majority having to vote a less-liked candidate over a more-liked one. That criterion is met by Condorcet, Approval, and Bucklin. It's not met by Instant Runoff (IRV or IRO), or by Schulze(Margins), or by any other method that measures pairwise defeats according to margins of defeat. Let's take it farther and ask that a majority have a way to ensure that someone won't win without any member of that majority having to vote a less-liked candidate _equal to_ or over their favorite. That criterion is met by the best Condorcet versions, including Schulze, SSD, SD, DCD, & Tideman. The methods that won't drop a defeat unless that defeat is the weakest defeat in some particular cycle. It's also met by Bucklin. All of the genuine Condorcet version, including the abovementioned ones, and Plain Condorcet & Smith Condorcet meet the criterion that asks that if a majority of all the voters vote the "Condorcet winner" (candidate who'd pairwise-beat everyone if all the voters sincerely ranked all the candidates--abbreviated CW) over candidate B, then B can't win unless people vote unfelt preferences. If that criterion is generalized so that there's no CW, but B, who isn't a member of the top cycle, is beaten by someone who is in the top cycle, and we don't want B to win if no one votes an unfelt preference, then now only the best Condorcet versions meet it, the versions that only drop a defeat if it's the weakest in some cycle--including the 5 methods I named in the previous paragraph. The 2 criteria described in the previous paragraph are about compromise strategy, because they say that, under certain plausible conditions, that majority can keep B from winning without doing anything other than ranking over him the CW or a member of the top cycle. These criteria will be more precisely stated in a subsequent message, in which I show why the best Condorcet versions meet the defensive strategy criteria, the majority-based criteria that include the ones that I listed here. To summarize, then, freedom from compromise strategy could be defined in a number of ways, some of which are met by no method; one met only by Approval; one met by a number of good methods including Approval, Bucklin & Condorcet (but not IRV or Schulze(Margins); and one that is met only by what I'll call the cycle Condorcet methods and Bucklin; and one met by all Condorcet versions; and one met only by the cycle Condorcet versions. Mike Ossipoff ______________________________________________________ Get Your Private, Free Email at http://www.hotmail.com
