[EM] The worst about each system; Approval Preferential Voting (new name for an MCA-like system)

[Kevin Venzke]

Hi Jameson,

I can't easily quote this message so I'll put my comments between symbols.


--- En date de : Mar 25.5.10, Jameson Quinn <jameson.quinn at gmail.com> a écrit :
What are the worst aspects of each major voting system?

-Plurality: Everything. It routinely requires dishonest strategy from a large minority, or even a majority, of voters. Enough said.

^
I think there happens to be a nice thing about Plurality in that it
is so primitive that everybody knows they have to have their act together
before the election.

There seems to be a feeling that this (pre-election consolidation) is 
completely bad, but I don't think it is. I mainly would like to have a
third choice.
^


-IRV: Voting can hurt you (nonmonotonicity). That means that small third parties can survive, but once they threaten to pass 25%, you're back to the problems of plurality. A great learning tool to understand this is http://zesty.ca/voting/voteline/ , which lets you play with one-dimensional scenarios and see how common nonmonotonicity is.

^
I don't call this problem nonmonotonicity... Even monotone methods can
have the situation that a third party candidate that is stronger than
expected can ruin the result and be unable to "fall back" to their
second preference that they didn't intend to have defeated.

Unless nonmonotonicity is politically unacceptable, or exploitable, I
don't think it's a problem.

The problem with IRV is that it eliminates candidates who have a good
chance of being good, in elections based in issue space.
^


-Condorcet: complexity. While the basic idea of one-on-one matches is simple, the details of tiebreakers are enough to make most voters' eyes glaze over. Moreover, the need to individually rank numerous candidates is more work than many are ready for, and the inevitable shortcuts they'll take could harm results.

^
I think complexity is one issue; another issue is the varying dangers
of strategy. I say issue rather than problem. The problem is if the issues
make it politically unacceptable.
^


-Approval: divisiveness. By forcing all votes into an all-or-nothing mold, it does not allow partial alliances between candidates. Consider the probable results in the recent Hawaii election, where the majority democrats split their votes between two candidates, leading to a Republican win. Lets assume for a second that, because the two democrats were distinguished mainly by individual not ideological factors, cross-party approvals are insignificant; and that Democrats are pretty evenly split between the two choices. Then, there are two possible results: either the less-cooperative Democratic faction wins, or, if the "uncooperative arms race" gets out-of-hand, the condorcet-loser Republican wins. In other words, the system has incentives not to cooperate between two frontrunners running approximately even in the polls, no matter how close they are, and these incentives are unhealthy whether or not they get out-of-hand.

^
Well, you don't have this problem only in Approval. I think the biggest
concern with Approval is that you can get a senseless result if the
voters have bad information.

It also doesn't easily admit three frontrunners. If you simulate polls
with left/right/center candidates, either left or right eventually
gives up (in my sims anyway) and votes for center, making two 
frontrunners, with center much more likely to win. Not a bad outcome
but it's still just two possible outcomes.

Sometimes with polls things never settle down, such as in the presence
of a cycle. In that case I find it hard to explain what the result was
really based on. The method here appears to be a game or tool like FPP
rather than a metric in itself, of who is a good candidate.
^

-Range: Strategy is too powerful. If one faction is more inclined to honestly rank, seeing themselves as neutral judges, while another faction has selfish reasons to strategically vote approval-style, the strategic faction will dominate, even if they are a minority. Range is very robust under strategy, if it's not factionally biased; but too vulnerable to factionally biased strategy. You can rationalize until you're blue in the face about how minority Range winners reflect a true societal preference; but imagine how you'd feel if Bush/Gore/Nader had been decided for your least-favorite against the will of the majority, due partly to a certain complicity of some people who should should SHOULD have been on your side, and partly to the obvious and dishonest machinations of the winning side, and you'll see that this is still a real problem. (OK, I know that doesn't take a lot of imagination for some people.)

^
For Range I would just say complexity, much in the same vein as your
criticism of Bucklin. They are basically Approval with ballots that offer
you unusual decisions.
^


-Bucklin: Bucklin (with equal rankings, of course) doesn't really have a single biggest weakness. It is still technically just as vulnerable to divisiveness as approval; but the trappings tend to hide this fact, and so it shouldn't be as much of a problem in practice. Still, it doesn't have any really strong points either. It's not the best honest system like Range; it doesn't give a Condorcet guarantee; and it's more complex than Approval, without really fixing Approval's greatest flaw.


So, allow me to restate my favored single-winner system, which, I think, avoids all of the major pitfalls above. I call it Approval Preferential Voting (the acronym, APV, is I believe only taken by American Preferential Voting, an old name for Bucklin; and since this system could be considered a Bucklin variant, I think that's just fine.)

Voters rank each candidate as preferred, approved, or unapproved. If any candidates have a majority ranking them at-least-approved, then the one of those which is most preferred wins outright. 

^
This part here has been thought of before: I/we called it MAFP. But
when no one had a majority then simply the approval winner would win.
^

If not, then the two candidates which are most preferred against all others (ie, the two Condorcet winners based on these simple ballots, or the two most-preferred in case of a Condorcet tie) proceed to a runoff.

^
This seems not clearly defined to me...? Are you saying the "two most
preferred candidates" and implying that this is the same as being the
two Condorcet winners?
^

This method is very simple. I think that the description above, without the parentheses, is simple and intuitive; it uses only concrete terms. It is also very easy for a voter to sort candidates into three rankings; I'd argue that this is the easiest possible ballot task, easier in general than either two or four ranking categories. (Two means too many compromises, and four means too many fine distinctions.)

^
Yes, I like this ballot format for C//A or ICA or Conditional Approval.
^

It's not quite the same as MCA or any other Bucklin system, since if there are two approval majorities, the preferences, not the approvals, break the tie. This makes APV more lesser-no-harm-like than Bucklin, encouraging voters not to truncate.

^
That's interesting, I'd never heard that argument before.

I did stumble upon this rule when considering the notion that the C
voters (if there are three blocs ABC in descending order of top prefs)
when voting C>A should not make B win when abstaining would let A be
the majority favorite: In Bucklin this isn't guaranteed, but if you break
the simultaneous majority using top rankings, it does work.


I'm having trouble understanding how monotonicity is guaranteed. Suppose
that there is a runoff between X and Y and X wins. Isn't it possible
that when X takes some preferences from Y, then instead the runoff is
between X and Z? Just like a normal runoff. Or is this move not 
considered because it's a three-slot ballot?

Those are the only comments I have at the moment.
^

Kevin