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

Tue Oct 18 22:52:54 PDT 2022

What if both systems could use rated ballots and allowed for ties (e.g. one
could vote for multiple "5-star" candidates, and multiple "4-star"
candidates, but could also use ranked ballots and allow for ties?  It seems
to me that conflating ballot type and tallying method is a mental hurdle
for folks, isn't it?

Rob

On Tue, Oct 18, 2022 at 10:22 PM Forest Simmons <forest.simmons21 at gmail.com>
wrote:

>  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.
>> > ----
>> > Election-Methods mailing list - see http://electorama.com/em for list
>> info
>> ----
>> Election-Methods mailing list - see https://electorama.com/em for list
>> info
>>
>
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