[Election-Methods] RELEASE: Instant Runoff Voting

[Kristofer Munsterhjelm]

(Oops, seems I sent this only to James Gilmour. Let's try again. )

James Gilmour wrote:

 >> it would have to look at the entire ballot.
 >
 > That is a consequence of your interpretation of how the voting system
 > is supposed to work and what the voting system is supposed to
 > be doing.  But that's not what IRV is about.  As I said in the
 > previous message, the origins of IRV are in the Exhaustive Ballot,
 > and in the Exhaustive Ballot there is no possibility of looking "at
 > the entire ballot".  IRV is not about satisfying a set of criteria
 > derived from social choice philosophy.

In taking the "people out of the loop" in all rounds but the first, the 
reduction of Exhaustive Ballot to IRV turns IRV into yet another ranked 
ballot method. Thus it wouldn't matter if IRV originates in Exhaustive 
Ballot or not, because it has to stand as a ranked ballot method among 
other ranked ballot methods, using criteria and tests that can be 
applied to all of them.

 > If you want something that only a social choice approach can deliver,
 > then clearly IRV is not for you.  But that does not make Kathy
 > Dopp's original statement a valid criticism of IRV.

Wouldn't it be, from a social choice point of view if no other?

 >> Or more concrete: if you want the sort of compromise that Condorcet
 >> gives (and you don't think that's a "weak centrist"), then you can't
 >> have LNHarm. I don't think you can have LNHelp either, but I'm not
 >> sure about that.
 >
 > I agree, but one could I think reasonably argue in the specific case
 > of Condorcet that it does comply with LNHarm (at least, in Condorcet
 > where there were no cycles or ties).  Your higher preferences are
 > always placed above your lower preferences in the Condorcet
 > "head-to-head" comparisons.  So YOUR lower preference can never harm
 > YOUR higher preference.  But that is certainly not true for many other
 > social choice voting systems that use the preference information in a
 > quite different way.

That's true; it's the cycles that cause the problem. Still, Woodall's 
proof shows that it's possible to make a ballot set with no CW in a way 
  that no matter who wins, it's possible to append a later preference to 
some of the ballots so that another candidate becomes the CW.
The problem is in the transition between cycle and non-cycle, so inasfar 
as Condorcet winners usually occur, the Condorcet method passes LNHarm; 
but since cycles can occur, that means Condorcet is incompatible with 
LNHarm.

If we look at it from what you call the social choice point of view, 
then what has happened that makes Condorcet fail LNHarm is that it's 
used a later preference to find the Condorcet winner that it didn't know 
of, had it only used earlier preferences.

 >>> "Many" on this list may think that, but it is my experience of more
 >>> than 45 years as a practical reformer explaining voting systems to
 >>> real electors, that 'later no harm' does matter greatly to ordinary
 >>> electors.  If they think the voting system will not comply with
 >>> 'later no harm', their immediate reaction is to say "I'm not going
 >>> to mark a second or any further preference because that will hurt my
 >>> first choice candidate  - the one I most want to see elected."  And
 >>> of course, if you once depart from 'later no harm' you open the way
 >>> to all sorts of strategic voting that just cannot work in a 'later
 >>> no harm' IRV (or STV) public election with large numbers of voters.

 >> If the method fails LNHarm about as often as it fails LNHelp,  then
 >> that argument should fail, because bullet voting may harm your other
 >> choices as much (or more, no way to know in general) as consistently
 >> voting all of them will. Ceteris paribus, it's better to have a
 >> method that passes both of the LNHs than neither (since you get
 >> strategy in the latter case), but the hit you take might not be as
 >> serious as it seems at first.
 >
 > Your argument in respect of bullet voting in IRV is based on a
 > misinterpretation of what that voter has said to the Returning
 > Officer.  Because IRV conforms to LNHarm, a bullet vote, or any
 > truncation, is a voter saying "After this point, I opt out and leave
 > any choice among the other candidates to the other voters."  Such a
 > voter has no "other choices".  So there is no question of harming them
 > or helping them.

That wasn't an argument against bullet voting in IRV. I know that IRV 
satisfies both LNHarm and LNHelp (it's also nonmonotonic, which is a 
consequence of that it satisfies both and Mutual Majority; but that's 
not relevant to the case here).

What I'm saying, regarding voting systems that fail LNH, is that you can 
divide strategies into those that every voter would use just to maximize 
the power of the ballot, and those that require information to pull off. 
If a voting system satisfies neither of the LNHs, and the rate of 
failure is balanced (doesn't consistently harm earlier candidates nor 
consistently help earlier candidates), then ordinary voters won't 
truncate (resp. randomly fill) because they don't know whether doing so 
would harm or help their candidate.

Sophisticated strategists would know, but if they don't have the lever 
granted to them by LNH failure, they would probably use some other 
strategy instead.

This is a simplification, since it may be the case that simple polls 
provide enough information, or that sophisticated strategy turns out to 
be very hard to manage (or have game-of-chicken dynamics), but it should 
show that the problem of failing LNH is not as serious as it might 
appear. It's worse than passing them, of course, but the LNHs are not 
criteria (in my opinion, at least) where a single failure dooms the method.