[EM] Simple Acceptable Ranked Choice Voting

Kevin Venzke stepjak at yahoo.fr
Fri Jan 13 05:37:53 PST 2023

Hi Forest,

Let's tackle this question first:

> Also how does DSC do with regard to Chicken Defense?

I'm glad you asked. DSC is a ""great"" CD method, maybe the only one.

I have complained before that the CD criterion allows the larger faction in a fragmented
majority to truncate without issue. That seems like a problem both philosophically (i.e.,
what is the significance of faction size in a chicken game?) and also practically (i.e. a
faction might truncate out of an erroneous belief that their faction is the large one).

Consider this election:

40 A
35 B
25 C>B

IRV and Condorcet methods go soft and let B win. Maybe B was driving a truck and C was
driving a car. But with DSC justice is blind. It stands its ground and elects A, handing
the fragmented majority a well-earned punishment!

And it goes much further:

100 A>B
99 B
98 C>B
97 D>B
... etc
50 Z>B

DSC will still elect A.

> I wonder how this DSC-with-max-gradient-finisher would do:
> Initialize X as the DSC winner. Then ...

I tried this with a few methods (DSC, DAC, FPP, approval) as I felt unsure you really meant
to pick DSC for this. However, I seem to find that generally, no matter the seed method,
this approach is violating monotonicity. Maybe this is creating ways to rig the initial
chain head.

> While X is covered, replace X with the candidate X' with the strongest defeat against X
> among those candidates that cover X.
> Elect the last value of X... i.e. the first X that turns out to be uncovered.
> Does any of the DSC burial resistance flavor make it through this afterburner?

DSC isn't particularly burial-resistant. Its significance is in lacking truncation
incentive. But since it likes to agree with FPP, that might moderate the burial issue.

As far as simulations:

Seeding with FPP or DSC made it a lot worse with minimal defense, so I can't say I like
those ones.

Some best-to-worst rankings for each seed type, and also C//A, with four candidates:

DAC > Approval > C//A > Gross score > DSC > FPP

FPP > DSC > DAC > Gross score > Approval > C//A

C//A > FPP > Approval > DAC > DSC > Gross score

For burial, FPP is the best seed probably because you can't manipulate the initial chain
head through adjusting your lower preferences.

In general it seems like the covering rule will introduce burial, because it gives voters
some levers to indirectly attack a potential chain head.


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