I agree that fractional ER-IRV is an inconsequential improvement over regular IRV.
I also have serious doubts about whole-vote ER-IRV, mainly over whether the Duvergerian equilibria would still be strong enough to maintain a two-party system. If so, then the differences between top-two, IRV, ER-IRV(fractional), ER-IRV(whole votes), or simply disqualifying all but the top-two primary winners from the general election, are largely academic, at least in U.S. partisan races. Bart Ingles Chris Benham wrote: > > James, > I have always regarded equal-rankings allowed IRV(fractional) as a > small, mostly irrelevant refinement of normal no equal-ranking > (except for truncation) allowed IRV which would never be implemented > because it makes counting more difficult (especially if > hand-counting paper ballots), and the demand for it from voters and > parties would be very small. > So,like you, I was pleasantly surprised to see that this seemingly > small refinement is a suffiiciently big improvement on standard IRV > for Mike Ossiopoff to rank it both above "Majority-Choice Approval" > (MCA) and Approval. According to Mike, it meets his > "Weak Defensive Strategy Criterion" (WDSC). From electionmethods.org: > > If a majority prefers one particular candidate to another, then they > should have a way of voting that will ensure that the other cannot > win, without any member of that majority reversing a preference for > one candidate over another. > > Judging by the example at Steve Eppley's site, it seems to meet his > (similar) "Non-Drastic Defense" criterion. I can't see or imagine any > possible theoretical disadvantage ER-IRV(fractional) could have > compared to standard IRV, so (in light of the above) I rate it as > unambiguusly better. > > The same cannot be said of ER-IRV(whole). Unlike standard IRV, it > fails the Symetric Completion criterion and the "No Zero-Information > Strategy" standard. The voter with no idea of how others vote, who > has a sufficiently large gap in his/her ratings, now does better to > insincerely rank all those candidates above the gap in equal-first > place. But that is far from the worst of it! > > Take this example of sincere preferences: > 45:Right>CentreRight>Left > 35:CentreRight>Right>Left > 20:Left>CentreRight>Left > > CentreRight is both the sincere CW and IRV winner. > IRV is vulnerable to the "Push-over" strategy. This from EMR: > > push-over > The strategy of ranking a weak alternative higher than one's preferred > alternative, which may be useful in a method that violates > monotonicity. > > In the above example, some (but not too many) of the Right supporters > can use the Push-over strategy to make Right win: > > 25:Right>CentreRight>Left > 20:Left>Right>CentreRight (these are Push-over strategising Right > supporters) > 35:CentreRight>Right>Left > 20:Left>CentreRight>Right > > Now CentreRight has the lowest first-preference tally, and then Right > wins. The strategists had to be sure that Right had a pairwise > win against Left, and that Right wouldn't be eliminated. It could be > difficult or risky to coordinate, because obviously if too many Right > supporters vote that way, then Left will win . > But look what happens when the method is ER-IRV(whole)! Now the Right > supporters have a vastly improved Pushover-like > opportunity. > > 45:Right=Left>CentreRight > 35:CentreRight>Right>Left > 20:Left>CentreRight>Right > > First-preference tallies > Right:45 CentreRight:35 Left:65 > > CentreRight has the lowest tally, and so is eliminated then Right > wins. > This time no coordination was needed. As long as the Right suporters > knew that Right had more first-prefernces than CentreRight, and a > pairwise win against Left, then each individual Right supporter got an > increased expectation by insincerely upranking Left from last to > equal-first with no risk. > This example wouldn't work if there was a "majority stopping rule" > (because then Left would be declared the winner on the first round), > but if there was, then we would have an Approval-like method with lots > of insincere compression incentive, that I am sure would fail > Clone Independence. > In the example, with ER-IRV(fractional) the same strategy by the > Right voters would also succeed, but the strategists had less margin > of error, and in general it is much easier and less risky with the > whole votes version. But contradicting what I wrote earlier, maybe it > is a > significant disadvantage of ER-IRV(fractional) versus plain IRV that > Push-over strategising is less risky and more tempting. > In conclusion, ER-IRV(whole) is worse than standard IRV. > ER-IRV(fractional) may be better than plain IRV, but I don't like its > chances of being introduced in practice. I would think that most > voters wouln't see much point in it, and election officials would hate > it. > > Chris Benham ---- Election-methods mailing list - see http://electorama.com/em for list info
