SP Voting: Explanatory Video
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@andy-dienes that just isn’t true. A very slim majority cannot guarantee their victory in SP Voting, it does not satisfy the majority criterion. Especially if the rank order variant is used. If you tried to actually explain how a majority might accomplish that, you would run into issues.
What I imagine you think would work is bullet voting. That’s impossible with a rank order variant. In any case, let’s say there are at least three candidates, and a slim majority aligns behind their first choice and last choice in an attempt to veto a candidate. Then all of the other candidates already have at least 50% of the electorate giving them no minimal scores. Then there are two cases:
First, if the majority first-choice is highly divisive, a large fraction of the non-majority is likely to score them minimally. This guarantees that there is some candidate with a broad supermajority of the electorate not giving them any minimal scores, and that candidate will almost surely prevent the highly divisive majoritarian candidate from winning the election.
Otherwise, if the majoritarian first choice is not highly divisive, then the system will be responding to the preferences of a supermajority, which is exactly what was desired in the first place.
In other words, the only way a majority can guarantee their first choice to win is either if they are also supported by a significant fraction of the minority, meaning that the majority top choice is not highly divisive.
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51% of voters rate candidate Alice 5/5, every other candidate 0/5
Unless I am misunderstanding the method, please explain how any other candidate can win in this scenario.
That’s impossible with a rank order variant
Are you suggesting to disallow equal ratings?
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@andy-dienes yes, the possibility of disallowing equal ratings was one of the first points addressed in the video. That turns SP Voting into a rank order system.
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@cfrank Even in the context of truncation? So you will require voters to rank every single candidate? This opens up a whole new array of issues
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@andy-dienes Of course, but it can be addressed in the same ways it would be addressed in any other system where that is a potential issue, such as Condorcet methods.
Another variant is to simply use the ballots themselves to define a discrete distribution.
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@cfrank said in SP Voting: Explanatory Video:
where that is a potential issue, such as Condorcet methods
but it is not a potential issue in the same way. you are imposing forced strict rankings as a band-aid for an issue with the method. Condorcet methods typically function just fine with equal rankings allowed.
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@andy-dienes no, it isn't a "band-aid," it's how the method can be defined to operate. SP Voting is a framework, it isn't a single method. By that logic any aspect of any voting system can be called a "band-aid." Disqualifying invalid ballots is always a necessary aspect of a voting system, and I'm sure there are reasonable defaults that can be defined to accommodate voters who don't want to rank all of the candidates, such as distributing their vote for unmarked candidates uniformly across all of the (unmarked) scores.
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SP Voting is a framework, it isn't a single method.
My perception is that when I give a criticism of one possible instantiation of SP you are pointing to a different instantiation and claiming it does not have that issue.
Even if the 'framework' includes enough different variants such that you can always choose one of the variants without some particular issue, you cannot just assert that thus SP automatically gets the best of all possible worlds and is free of issues. This is moving the goalposts. By that logic, why not just propose that your "framework" is all possible voting rules?
If SP allows equal rankings, it has (in my view) major issues. If it does not allow equal rankings, then it has different major issues. Can you pick a particular instantiation and we can continue the discussion from there?
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@andy-dienes SP Voting definitely does not automatically get the best of all possible worlds, nor is it free of issues. The issues you are raising are valid, and I'm glad you are raising them. I want to see if the parameters of the framework can be adjusted to accommodate them not just individually but all together.
The SP Voting methods I'm proposing are mostly for theoretical comparison purposes. I think the ranked order variant would give very good results, but as you mention, it has the issue of forcing complete rankings. What I'm curious about is, as we address these issues, whether the SP Voting framework converges in any sense to some kind of more fully specified voting system. And then, in that case, maybe the system can be simplified to capture the desired behavior. If it ends up being something like a Condorcet method, that would be interesting to me! That would pretty much convince me that Condorcet methods, which I am personally on the fence about, are the way to go. I still think that Condorcet methods could be relaxed a bit to stray away from majoritarianism toward a more consensualistic mechanism, but who knows.
For right now, if you're interested in considering this system, I'm thinking of an intermediary variant that does not require full ranking and allows equal scores: Let's say for now that voters can only indicate a single candidate in the minimum score position, and the default score for an unmarked candidate is S1 (unless perhaps they are unmarked by a supermajority (say 2/3) of the electorate, whereby they will be disqualified). I am still considering the geometric weighting with a common ratio of 1/2.
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@cfrank said in SP Voting: Explanatory Video:
for now that voters can only indicate a single candidate in the minimum score position
This will introduce pretty severe teaming strategies in the same vein as Borda's.
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@andy-dienes yes, that makes sense. Alternatively, one might require that for every minimum score on a ballot, there is at least one corresponding non-minimum score. That would lessen the effect of teaming but also give a majoritarian advantage.
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@cfrank Then whichever party has more candidates than the other is advantaged. This makes the clone failures even worse