[EM] Couple of comments (AMS, Why the fuss)

[Kevin Venzke]

Hi Kristofer,

Le lundi 6 mars 2023 à 10:25:05 UTC−6, Kristofer Munsterhjelm <km_elmet at t-online.de> a écrit :
> > I recently made a calculator that tries to show the steps to solve Ranked Pairs and River
> > and I couldn't come up with a satisfying way to dedupe all the possible ways that one could
> > traverse the propositions when there are tied strengths, and potentially many. I settled on
> > showing up to two traversals per possible winner.
> 
> IIRC, Tideman originally proposed a method where X wins if there exists
> a way to break ties so that X comes unambiguously first in Ranked Pairs.
> This method is neutral, but it's also (as Colin said) NP-complete. In
> practice, people just break ties in some way. I think that the AMS
> method (just a coin flip) fails clone independence and that random voter
> hierarchy, while cloneproof, fails summability. But I'm not entirely
> sure about either.

That sounds right. Additionally, I think you can't escape that if you say "We had to flip a
coin at step 4" or something, people will probably demand to know whether that determined
the outcome and if so which other candidates might have won.

> (E.g. part of my intuition says: "if you pick a random voter and rank
> A>B over B>A if that voter ranks A over B... isn't that the same as a
> coin toss? Because imagine you eliminate everybody but A and B, then the
> number of voters who rank A above B is just A>B, and the number of
> voters who rank B above A is B>A; and by presumption that there's a tie,
> A>B must be equal to B>A, so the chance of picking a voter who prefers A
> to B is 50%". If it were that simple, then there would be no need for
> random voter hierarchy, so clearly I'm missing something.)

Candidates in an RP tie don't need to be in a pairwise tie. The situation can be that you
have tied strengths E>F and I>J and depending on which one gets considered first, some
other random candidate will win.

> > One thing I could do is provide a bunch of examples on what methods
> > look like in my framework, taking the form of a Python 3 class, and
> > then people could just send me methods if they want. I'm not sure if
> > there would be any takers for that. It would probably be more
> > interesting if I was maintaining and publishing a big list of
> > results, which at the moment I'm not. (Partly due to the sensitivity
> > of results to scenario parameters.)
> I've had similar ideas myself, that surely it should be possible to not
> be duplicating each other's efforts as much. Strictly speaking, my
> quadelect simulator is extensible, but I wouldn't call it very easy to
> use and C++ has its own quirks.
> 
> Would you be interested in putting your code on a version control system
> of some kind?

Not any time soon. It's easier to consider sharing some version privately, if you think it
would help your investigations. I'd still need to put some work into that though.

Kevin
votingmethods.net