[EM] Copeland//Plurality --- can it beat IRV?

[Kristofer Munsterhjelm]

On 22.01.2022 21:33, Daniel Carrera wrote:
> Hi guys,
> 
> As you know, like many of you I am dismayed that so many election reform
> advocates are promoting IRV, apparently thinking that it is the only or
> the best alternative to FPTP. So once again I'm trying to think of
> methods that might appeal to an existing IRV advocate, to see if I can
> get them to ditch IRV and pick something that is actually good. In the
> past you've seen me ask about BTR-IRV, Benham, Smith[//,,]IRV, etc. So
> let me present another idea:
> 
> What about Copeland//Plurality?
> 
> 1) The method is incredibly easy:  "Among the candidates that win the
> most head-to-head matches, pick the one with the most first votes."
> (yes, I'm setting it so the score for ties is zero)
> 
> 2) I'm pretty sure that the method is Smith efficient and monotone.

It probably fails monotonicity, because it's X//Y and it's not Borda.
The thing that makes Copeland//Borda monotone is that the pairwise
matrix doesn't change when you remove candidates. But the number of
first preferences for remaining candidates *does* change when candidates
are eliminated.

I think the scenario would be something like this: Suppose we have an
A>B>C>D>A cycle. Some voters uprank A which turn the D>A leg into A>D,
so that D is booted from the Copeland set. D-first voters tend to vote
for B second, and A has the most first preferences with B in second
place. After D is booted out, the revealed first preferences for B make
B win instead of A; so raising A made A lose.

I would also guess that it would be pretty susceptible to strategy. It
isn't cloneproof either because neither Copeland nor Plurality is.

> 3) For an IRV advocate it might have better intuitive appeal than other
> alternatives because it has that Plurality component that some of them
> want. I've read IRV advocates say that IRV > Condorcet because the
> winning candidate should have strong 1st-place support. I can't imagine
> any reason why that would possibly be true, but it means that an
> X//Plurality method might appeal to them.

I always find the core support argument kind of weird. If core support
is the end-all be-all, then Plurality's your man. But clearly it isn't.
On the other hand, IRV can elect a candidate who has only two first
preferences, so the worst case (lack of) reliance on core support can be
pretty bad -- and IRV can fail to elect the CW even when the CW has more
initial first preferences than the IRV winner. So it doesn't really hold up.

-km