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

[Kristofer Munsterhjelm]

On 19.10.2022 05:50, Rob Lanphier 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.

One way of looking at it is that Range and STAR are about honest 
performance, whereas fpA-fpC is about resistance to strategy.[1] So 
whether one is better than the other depends on what you prioritize.

More concretely, fpA-fpC and its variants attempt to retain IRV's 
strategy resistance while fixing its chaos and monotonicity problems and 
making it Smith in a natural way. They resist strategy by passing 
DMTCBR, which seems to grant considerable burial resistance in practice.

(As a bit of a happy coincidence, both Friendly Cover and Forest's 
Friendly Voting also happen to be summable, which Smith-IRV is not. I 
know that rb-j has emphasized summability in his Condorcet advocacy.)

The fpA-fpC variants are mainly research methods. But research methods 
can turn out to be useful in practice: Tideman's Ranked Pairs was also 
originally a research method. A suitably advanced fpA-fpC variant could 
be useful where strategy resistance is very important but IRV is 
recognized as just too flawed.

As examples of strategy resistance, fpA-fpC and its generalizations pass 
the chicken dilemma criterion, thus avoiding the Burr dilemma of plain 
Range (though one could argue that chicken dilemmas aren't as severe to 
begin with for ranked methods). Friendly Cover is also independent of 
twins - cloning candidates one at a time - while STAR is not.[2]

Finally, I'm limiting myself to ranked ballots partly because I think 
rated ones don't have good enough a foundation. There may be better 
strategy-resistant rated methods, but I haven't looked. [3]

-km

[1] James Green-Armytage shows that Range is considerably more 
susceptible to strategy than Smith-IRV (Condorcet-Hare), see 
http://jamesgreenarmytage.com/strategy-utility.pdf page 18. 
(Unfortunately he didn't include STAR.)

[2] This probably holds for Friendly Voting too, but I haven't checked 
it yet.

[3] For instance, Smith//Range or Smith,Range would be cloneproof by 
rated standards; perhaps there exists a monotonicity-fixed version of 
renormalized Smith//Range, the way fpA-fpC is a monotonicity-fixed 
version of Smith-IRV. Or something based off SARVO-Range.