[EM] Fwd: Fwd: U/P voting: new name for simple 3-level method.
stepjak at yahoo.fr
Mon Sep 12 17:22:12 PDT 2016
I think it is a positive thing that the MTA B/C majority coalition can give their sincere preferences (!), while using the strategy they're expected to use (i.e. middle slot as tiebreaker given multiple majorities), without risk of this strategy backfiring. (Voters can accidentally elect the less preferred of B or C, but that is the inescapable chicken dilemma, I would say.)
I have some sympathy for your claim that C should not be able to win with few top ratings. But that sympathy is not tied to Borda counts, it is based on wanting to reduce the truncation incentive for the B voters. This, U/P does not really do, because the B>C voters would be taking a large risk that they are helping to put C (alone) over the threshold of majority approval.
So I don't think either of these ballot sets is likely under U/P, and it sounds like you agree with that and think it is good (because it deters a pathological ballot set)? Do you have a stance (or at least, see use in determining a stance) on how U/P voters in these scenarios should be voting?
De : Jameson Quinn <jameson.quinn at gmail.com>
À : EM <election-methods at lists.electorama.com>
Envoyé le : Dimanche 11 septembre 2016 1h51
Objet : [EM] Fwd: Fwd: U/P voting: new name for simple 3-level method.
2016-09-10 21:26 GMT-04:00 C.Benham <cbenham at adam.com.au>:
On 9/11/2016 5:02 AM, Kevin Venzke wrote:
Under MTA the B and C voters are being completely reasonable: They hope for majority approval but can still hope for a win if they
don't get it.
Strategy is less likely to produce these ballots under U/P because the B and C voters are taking a gamble. To get a similar outcome
they have to vote B=C. Anyone who doesn't is functionally defecting!
C: A very good example! Assuming MTA and MCA use Top Ratings scores to break Approval ties, they both elect the Condorcet winner B.
But both could be shifted to C with a single C-only ballot, even if the B:C ratio were 46:1 instead of 24:23.
U/P's under-use of the middle ratings slot means that it relies more on its "majority disqualification" mechanism which seems to make it more
vulnerable to irrelevant ballots, as in the example.
Under U/P, without the irrelevant D ballots, A and D are disqualified and B is the glorious winner. With them, B and C and D are disqualified and (without needing
any others to be disqualified) A wins.
This causes me to reject U/P as clearly worse than MTA and MCA. Of the three I (again) rate MTA as the least bad.
I think MTA is pretty darn good. I still prefer U/P.
I think that scenarios like the above are fundamentally pathological; any possible winner has only minority approval, so that even assuming all ballots are semi-honest, any of them could be a true Condorcet loser. Thus, I believe that it's more important for a system to try to avoid scenarios like the above, than to try to find a perfect winner in such a scenario. In fact, in the related scenario:
40: B>C6: C>B
... I think that a case can be made for either A or B. After all, they'd be tied if we try to approximate Score by using truncatable Borda here. But no serious case can be made for C or D, even though C wins MTA and MCA.
Anyway, I think U/P does a better job trying to discourage the kind of strategy that would lead to a scenario like the above. And part of that is the default rule which Chris has criticized.
One possible alternative default rule: ballots alternate between defaulting to "acceptable" and to "unacceptable". Each ballot clearly states which default it uses, and there is a place on the ballot to globally change that default. (I doubt Chris will like this idea, but it is at least straightforward, explicit, and easy to describe.)
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