[EM] fpA-fpC vs STAR (was "Re: Strategy-resistant monotone methods")

Tue Oct 18 22:22:24 PDT 2022

 Suppose you had two methods that were indistinguishable in merit,
including simplicity and transparency, except that one was based on Score
ballots, and the other was based on Ranked Choice ballots. Which one (all
else being equal) would be the easier sell?

On Tue, Oct 18, 2022, 8:51 PM Rob Lanphier <roblan at gmail.com> wrote:

> Hi folks
>
> I'm going to revive a REALLY old thread, which is where Kristofer
> Munsterhjelm first describes "fpA-fpC", which is described here:
>
> https://electowiki.org/wiki/fpA-fpC
>
> At some point, I'll set myself up for simulations, but for now, I'm
> going to take the REALLY LAZY approach, which is to ask on this
> mailing list without really reading all of the old EM threads and
> understanding what y'all were saying.
>
> The main question I'd like to ask: what is it that makes fpA-fpC a
> better method than STAR?  Certainly not the name, but Kristofer hasn't
> given up on it (it would seem [1]), so SOMETHING must be better about
> it. What is it?  I'm most interested in how it compares to STAR
> because STAR seems to have a lot more traction than any
> Condorcet-winner-criterion (CWC) compliant methods that I'm aware of.
> I have a soft-spot for CWC-compliant methods, but I've been convinced
> that strict CWC isn't necessary as long as a system is close enough.
>
> Rob
>
> [1]: As evidence he hasn't given up, Kristofer's still adding to the
> "fpA-fpC" page on electowiki:
>
> https://electowiki.org/w/index.php?title=FpA-fpC&curid=2805&diff=16412&oldid=15820
>
> On Fri, Feb 19, 2016 at 12:25 PM Kristofer Munsterhjelm
> <km_elmet at t-online.de> wrote:
> >
> > On 02/09/2016 03:57 AM, Kevin Venzke wrote:
> > > Hi Kristofer,
> > >
> > > Thanks for the additional details.
> > >
> > >
> > > ----- Mail original -----
> > > De : Kristofer Munsterhjelm <km_elmet at t-online.de>
> > > À : Kevin Venzke <stepjak at yahoo.fr>; EM list <
> election-methods at electorama.com>
> > > Envoyé le : Dimanche 7 février 2016 16h52
> > > Objet : Re: [EM] Strategy-resistant monotone methods
> > >>>>
> > >>>> The best simple linear method I could find was this:
> > >>>>
> > >>>> f = fpA - fpC
> > >>>>
> > >>>> i.e. a candidate's score is the number of first preferences he has,
> > >>>
> > >>>> minus the number of first preferences for whoever is beating him
> pairwise.
> > >>>
> > >>> I think it's clear why this works: the candidate C who beats A
> doesn't get
> > >>> "credit" for all C>A votes but only those dedicated to C as first
> preference.
> > >>> So, the effect of strategic B>C>A votes (where sincere is B>A>C) is
> limited
> > >>> to causing a cycle.
> > >>
> > >> Right, it feels a bit like a restricted tactical position in chess,
> > >> where you know what you need to accomplish, but you can't get your
> > >> pieces around in time. In other words, there's not enough freedom for
> > >> the strategic voters to do everything they want to do at once.
> > >>
> > >> So intuitively I can see how it works. But I was hoping it'd be
> possible
> > >> to derive some kind of theory of methods resistant to strategy, and
> that
> > >> intuition doesn't seem to help us much; unless the best way of making
> a
> > >> method resist strategy is reducing the freedom in a way analogous to
> the
> > >> above.
> > >
> > > That's an interesting way of looking at it. I'm not sure if we could
> easily
> > > generalize this reduction of freedom concept though.
> > >
> > > I see the issue much more simply, that the effect (or perhaps
> "independence")
> > > of lower preferences needs to be kept low, on the assumption that they
> might
> > > be mischief votes. (The possibility that they might NOT be mischief
> votes
> > > usually limits my enthusiasm for these efforts!)
> >
> > That'd seem intuitive, but neither C/Plurality nor C/Antiplurality do
> > very well, resistance wise. The simulation provides the following
> results:
> >
> > [Condorcet],[ER-Plurality]:
> >  Impartial Culture: susceptible 78269/100000 = 78.3% of the time, 1547
> ties
> >  Gaussian: susceptible 21595/100000 = 21.6% of the time, 80 ties
> >
> > [Condorcet],[ER-Antiplurality]:
> >  Impartial Culture: susceptible 47579/100000 = 47.6% of the time, 1957
> ties
> >  Gaussian: susceptible 20841/100000 = 20.8% of the time, 91 ties.
> >
> > Plurality passes both LNHelp and LNHarm because it doesn't care about
> > later preferences at all. Yet it does badly even when its vulnerability
> > to compromising is reduced by prefixing it by Condorcet. Its IC
> > susceptibility is greater than the 75% of the advanced reversal
> > symmetric Condorcet methods even though the latter meet neither LNHelp
> > nor LNHarm.
> >
> > > In this scenario, when you have decided in advance that it's going to
> be a
> > > Condorcet method, you necessitate that some mischief is going to be
> possible.
> > > That's inherent to Condorcet, and the nature of the vulnerability is
> > > basically the same no matter what you do. So all you can do is try to
> reduce
> > > the damage done within the method of cycle resolution.
> >
> > I agree with this. Condorcet implies some vulnerabilities (and some
> > incompatibilities in general, e.g. it's impossible to get both
> > Participation and Condorcet). So the method can be resistant in two
> > ways: either resistant on its own inside the cycle regime, or resistant
> > in a way that meshes with the Condorcet completion (i.e. strategy X
> > would ordinarily be possible, but trying to execute it makes someone you
> > don't want into the CW).
> >
> > I also think that you're right about C,IRV working because IRV works.
> > The main thing Condorcet gives to IRV is compromising resistance (and
> > some degree of clone resistance, IIRC; see JGA's paper). You lose some
> > burial resistance and gain some compromising resistance, and in IRV's
> > case, the trade seems to be worth it. But the question then becomes why
> > IRV works yet Plurality fails.
> >
> > > I guess that doesn't cover all the bases, only burial. I doubt
> push-over is
> > > really a big deal; in my own simulations I don't recall any sensible
> methods
> > > with lots of push-over incentive.
> >
> > I see push-over more as a problem that hurts honesty than a strategy as
> > such. If voters find out that they could have had X if only they ranked
> > him lower, they're understandably going to be upset. Since IRV is
> > chaotic, it'd be very hard to actually use push-over to cheat or do harm.
> >
> > One of the reasons that I started with the whole search was that I
> > suspected that nonmonotonicity was something you'd just have to endure
> > to get strategy resistance - kind of like Random Ballot in that if you
> > want resistance, you're going to have to scramble the field so it's
> > either hard or impossible for strategists to find a consistent strategy.
> > But the search proved me (somewhat) wrong, since the fpA-fpC method is
> > monotone yet resists quite well. If I had done more thorough research
> > before starting, I'd also have remembered that Carey is monotone when
> > limited to three candidates and that C,Carey is also quite resistant.
> > ----
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> info
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> info
>
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