[EM] IRV vs RCV

[Rob Lanphier]

On Sat, Dec 14, 2019 at 7:21 AM robert bristow-johnson
<rbj at audioimagination.com> wrote:
> > On December 14, 2019 2:44 AM Rob Lanphier <robla at robla.net> wrote:
> > I also like Ranked Pairs, but
> > I suspect that all of the methods that pick a candidate out of the
> > Smith set are indistinguishable in real-world conditions.  My hunch is
> > that an analysis of the public elections that had ranked ballots would
> > reveal that all of them had a single Condorcet winner,
>
> except, of course, Burlington Vermont 2009.

I'm pretty sure all of the Condorcet-winner compliant methods chose
Andy Montroll, given the ballots from the Burlington 2009 election.
Copeland, Schulze, Ranked Pairs,  etc.  Was there a discrepency
between Condorcet methods, or just the well-documented discrepency
between the Condorcet methods and IRV?

> > and therefore
> > there would be no difference between the results of Ranked Pairs,
> > Schulze, Tideman, Schulze, or even Copeland.
>
> this is the difficult point i have tried to say here.  i think that Schulze is likely the most resistant to voting strategy, but it's too difficult to explain to legislators and the public.

Yeah, I agree.  I'm willing to take it on faith that BTR-STV is more
susceptible to strategy than methods that guarantee Smith set
membership, but I suspect that Condorcet-compliant methods perform
better at strategy resistance than standard IRV does.


> BTR-STV is different.  Schulze, RP, MinMax (dunno about Copeland) all elect the same candidate in the case of an CW or a Smith set of 3.  i don't ever ever ever expect to see a Condorcet RCV ever have a Smith set larger than and i really don't expect to see one without a CW.

Copeland isn't guaranteed to pick a candidate out of the Smith set
when the Smith set is bigger than one, so it's possible it'll pick a
different winner than Schulze, RP, MinMax, etc when the Smith set is
3.

> >  So BTR-STV seems like a
> > fine compromise, since IRV has failed to pick the Condorcet winner in
> > at least one recent public election.
>
> yes, and i am trying to remind the Progs of that.  but they are not listening.

*sigh*.  Yeah, sounds tough.  We had a close mayoral election here in
San Francisco in 2018.  Given how close it was, I was really terrified
that we'd end up with an election like Burlington 2009.  Thankfully,
the IRV elimination order didn't threaten to eliminate the Condorcet
winner.  The closeness of the race was between two candidates who
probably would have been the final two candidates in a BTR-IRV tally
(though the third place candidate wasn't far behind either of the
frontrunners).  Given the closeness bitterness of the race, it would
have been an electoral reform disaster if any of the top three
candidates had lost the way that Andy Montroll did in Burlington (as
the Condorcet winner and IRV loser).

Rob