[EM] Re: equal rankings IRV

[Bart Ingles]

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