[EM] Simple Acceptable Ranked Choice Voting

[Kevin Venzke]

Hi Forest, I may give it a look, but a couple of issues.

One thing, I feel a little unsure it will be monotone to pick a new candidate to add to the
chain based on who the current head is. Normally these methods don't actually depend on the
ordering of candidates already in the chain.
 
> It is well known that not all nice properties are compatible with each other in a common
> single winner voting method. However, we shall see by example that the following nice
> features are mutually compatible in a simple RCV voting method (MGCB defined below).
> Therefore, excluding any of them can only be justified by trading in the excluded ones for
> equally important ones in an equally simple method.
> 
> 1. The method should be clone independent like IRV ... therefore not plagued by the spoiler
> problem like First Past The Post Plurality or by "teaming" like the Borda Count.

Generous of you to imply that IRV is not plagued by a spoiler problem.

> It was a comment of Kristofer that inspired this method. He mentioned that according to his
> recent simulations electing the winner of the over-all strongest defeat A>B is a
> surprisingly burial resistant stand-alone method.  It seems to me that the burial resistance
> should carry over to this MGCB completion of his discovery.

Well. That kind of method is a stone's throw away from just being approval.

> Furthermore, it appears that if defeat strength is gauged by margins, then the method is
> Chicken resistant.

If you mean "more" chicken-resistant then OK, but if you mean satisfying the CD criterion,
there's no reason for that to be true. If you want to guarantee CD you need to deliberately
reject some majorities (which is completely in its spirit), not simply be indifferent to
them.

More importantly, I would point out that from a CD criterion standpoint, the Alaska RCV
outcome was probably completely correct.

It seems to me the Alaska race demonstrates that the CD criterion can't deliver on its
promise even under IRV, its most advantageous setting. It's not going to be better under a
method that poses actual incentives to withhold lower preferences.

> A fairly simple modification ... where the chain is built up from both ends ... always
> giving priority to the end where the new defeat is stronger .... preserves all six of the
> nice features in our list while adding a Strong Reverse Symmetry feature ... reversing all
> of the ballot rankings precisely reverses the output chain .... swapping the head and tail
> of the completed chain.

I don't follow. Are you just saying there's a choice of whether you beat the head or the
tail, but in either case the newly added candidate becomes the new head?

Kevin
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