[EM] IRV vs RCV

[Rob Lanphier]

Kristofer, I stand corrected on both of the points I was trying to
make (thank you!).  More inline:

On Tue, Dec 17, 2019 at 4:02 PM Kristofer Munsterhjelm
<km_elmet at t-online.de> wrote:
> On 17/12/2019 08.12, Rob Lanphier wrote:
> > On Sat, Dec 14, 2019 at 7:21 AM robert bristow-johnson <rbj at audioimagination.com> wrote:
> > 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.
>
> Doesn't BTR-IRV pass Smith? Suppose that X is in the Smith set and Y is
> not. Once X and Y meet in the bottom-two runoff, then by definition of
> the Smith set, X beats Y pairwise, so Y is eliminated.

Oh, that's delightfully simple!  Your informal proof of BTR-IRV
passing Smith seems correct to me.

I'm now struggling to figure out what the practical benefits of the
other Condorcet methods over BTR-IRV.  Given that BTR-IRV is
reasonably simple to explain, it has an intuitive connection to IRV,
it's hard to understand what the practical benefit is to advocating
for other Condorcet-winner compliant systems.

> > 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.
>
> That also seems wrong. See theorem 1 of
> http://dss.in.tum.de/files/brandt-research/choicesets.pdf.

Based on what I learned about Copeland back in 1996 when I was first
learning this stuff, I somehow dismissed the usefulness of the
Copeland set, and exhalted the use of the Smith set (since
Smith//Minmax(wv) seemed to be the preferred method discussed on EM
back in 1996, as I recall).  That paper looks like something I should
spend more time reading.

Rob