[EM] "Independence of cycles" and a possible new method.
km_elmet at t-online.de
Sun Dec 12 03:38:20 PST 2021
On 12/12/21 11:04 AM, Toby Pereira wrote:
> If you look at the example here:
> <https://www.rangevoting.org/TobyCondParadox.html> having a tie cycle in
> a Condorcet election can change the result. The ballots are essentially:
> 1 voter: A>B>C
> 2 voters: B>A>C
> 2 voters: A>B>C
> 2 voters: B>C>A
> 2 voters: C>A>B
> A is the Condorcet winner in this election, beating both B and C by 5
> votes to 4. However, in the top section of ballots, B is the Condorcet
> winner, and the bottom section is a three-way tie as it is a perfect
> cycle. Adding in tie cycles to change the result could be seen as a
> failure of "independence of cycles" (or perhaps there is a better name).
> This is also a specific case of failure of consistency. Or maybe a weak
> failure because B doesn't win both the sub-elections. B would have an
> outright win from the top ballots and tie from the bottom ballots, but
> loses to A when all are combined.
> This example, by the way, is a simplified version of something that
> Donald Saari used to promote the Borda Count.
> The problem is that in an election with more than three candidates, you
> couldn't simply remove the cycles and calculate the result. Ballots and
> candidates would potentially be involved in many intertwined cycles, so
> there would be no straightforward way of doing it.
> But what you can do is compare every possible triplet of candidates
> (like Condorcet methods compare pairs). For each triplet, all tie cycles
> are removed and you look at the head-to-heads.
You could do this, but as I understand the example, the method would no
longer be a Condorcet method. You could also define the irrelevance of
cycles criterion, perhaps something like:
Removing a constant number of voters who together form an exact tied
Condorcet cycle should not modify the output.
Though I'm not sure what the implications would be - or if it's possible
to pass by any method that fails IIA.
As for using triplets instead of pairs, I think Stensholt suggested that
doing so might be a way to generalize his BPW method, which is only
defined for three candiates. Similarly, it might be a way of
generalizing my fpA-fpC, though I'm again unsure how to do so and
preserve the desired properties of DMTBR and monotonicity.
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