[EM] Manipulability stats for (some) poll methods
Chris Benham
cbenhamau at yahoo.com.au
Sun Apr 28 16:14:05 PDT 2024
> Michael—you're right that it means favorite-burial (cutting the "head"
> off a ballot). The term is quite old, though (older than "favorite
> betrayal" or "favorite burial" I believe)
> <https://www.cambridge.org/core/product/identifier/S0003055400194481/type/journal_article>.
I don't like either term. For me, "burial" refers to something a voter
does to a candidate in the hope that will cause that candidate to lose
to a candidate the voter prefers, and not just to any insincere
down-ranking. So "favorite-burial" is an oxymoron that Mike O. likes
to use.
"Favorite Betrayal" meaning to insincerely down-rank one's favourite, is
ok, but that could either be Compromise strategy (insincerely up-ranking
X to decrease the chance that X will lose to a candidate you like less)
or Push-over (insincerely up-ranking X to increase the chance that X
will lose to say F that you like better, instead of F losing to some Y
that you like less).
An attempt was made to standardise the terminology here quite a while ago:
http://condorcet.org/emr/defn.shtml
> *burying*
> Insincerely ranking an alternative lower in the hope of defeating it.
> *compromising*
> Insincerely ranking an alternative higher in the hope of getting it
> elected.
> *push-over*
> The strategy of ranking a weak alternative higher than one's preferred
> alternative, which may be useful in a method that violatesmonotonicity
> <https://web.archive.org/web/20090613041320/http://condorcet.org/emr/defn.shtml#monotonicity>
>
> *monotonicity*
> The property of a method where an alternative can never be made to
> succeed by being ranked lower on some ballots. Doing this is using
> the "push-over
> <https://web.archive.org/web/20090613041320/http://condorcet.org/emr/defn.shtml#push-over>"
> strategy.
>
>
The genius of STAR Voting is that it apparently technically doesn't
"violate monotonicity", but it is vastly more vulnerable to Push-over
than IRV which does, as I've explained previously.
I find the question of whether there have been mistakes or exaggerations
or false claims made by the IRV promoters to be completely irrelevant
to, and separate question from, whether its adoption in the US should be
supported.
Of more concern to me are the details of the ballot rules and
restrictions. I think it is more democratic for it to be relatively easy
to get on the ballot so as to allow the voters a wider choice of
candidates. I understand that typically voters are limited to 7
different "ranking levels". Well say there are 9 candidates and my two
least-preferred candidates are the two front-runners and I have a
preference between them. If I vote sincerely my vote is just as wasted
as if I had voted sincerely in FPP.
IRV then doesn't have Clone Independence. One of the main points of
ranked-ballot versus FPP is to reduce the involuntarily wasted vote as
much as possible.
If such restrictive ballot rules are unavoidable, then I lose my
enthusiasm for IRV or Benham in favour of something with a truncation
incentive (and that is happy with equal-ranking if that isn't a problem
for the ballot rules) such as Smith//Approval (implicit).
Properly implemented Hare's Compromise incentive is practically nothing
by comparison with that of FPP, and no-one in Australia notices it. I
estimate it is also quite a bit weaker than that of STAR. Pushover
strategy in Hare is relatively difficult and risky in Hare and as far as
I know it's never been tried in Australia. Whereas STAR is a "festival
of Push-over" farce/nightmare.
And while (like STAR) it fails Condorcet, it has a solid set of
"representativeness" criterion compliances that together can be thought
of as weakened Condorcet and are worth quite a lot.
They are Dominant Coalition (a better stronger version of Mutual
Majority that so of course implies it) and Dominant Mutual Third and
Condorcet Loser. STAR only meets the last of those, the weakest.
Chris B.
On 29/04/2024 5:08 am, Closed Limelike Curves wrote:
> Michael—you're right that it means favorite-burial (cutting the "head"
> off a ballot). The term is quite old, though (older than "favorite
> betrayal" or "favorite burial" I believe)
> <https://www.cambridge.org/core/product/identifier/S0003055400194481/type/journal_article>.
>
> On Sun, Apr 28, 2024 at 12:02 PM Michael Ossipoff
> <email9648742 at gmail.com> wrote:
>
> Of course I’m just guessing, but my guess is that “decapitation”
> is Closed’s new name for favorite-burial.
>
> Closed sometimes in invents new names without define them.
>
> IRV indeed shares Plurality’s need for favorite-burial
> defensive-strategy. I don’t like that, & wouldn’t propose IRV.
> There are a number of places where IRV is (the only electoral
> reform) up for enactment this year, In spite of that very
> unlikeable strategy-need, I wanted to help campaign for its
> enactment, in the hope that the voters who’ve enacted it didn’t do
> so because they intend to bury their favorite, & so so won’t do so.
>
> But, because IRV is being fraudulently sold to them, with
> intentional lies, we can’t count on how people will vote when they
> find out about what they’ve enacted…when they find out about the lie.
>
> Therefore, regrettably, we shouldn’t support “RCV”.
>
>
> On Sun, Apr 28, 2024 at 11:15 Chris Benham
> <cbenhamau at yahoo.com.au> wrote:
>
> Limelike,
>
> Can you please define and explain the "decapitation"
> strategy? I haven't heard of it.
>
> And can you elaborate a bit on this? :
>
>> IRV is a good example of this. It's /usually/ not susceptible
>> to strategy (in the IAC model), but I think of it as one of
>> the most strategy-afflicted methods on this list. It's
>> vulnerable to some particularly-egregious strategies
>> (decapitation), ones that are complex or difficult to explain
>> (pushover), and many strategies [that?] don't have a simple
>> defensive counterstrategy available (like truncation).
>
> Chris B.
>
> On 29/04/2024 2:31 am, Closed Limelike Curves wrote:
>> Hi Kris, thanks for the results! They're definitely interesting.
>>
>> That said, I'm not sure how useful a metric raw
>> probabilities provide; I don't think they provide a very
>> strong measure of how /severely/ each system is affected by
>> strategy. Missing are:
>> 1. How much do voters have to distort their ballots? Is it
>> just truncation, compression (as with tied-at-the-top), or
>> full decapitation?
>> 2. How hard is it to think of the strategy? Counterintuitive
>> strategies (e.g. randomized strategies or pushover) require
>> large, organized parties to educate their supporters about
>> how to pull it off. This could be good or bad depending on if
>> you like institutionalized parties. Good: sometimes people
>> can't pull it off. Bad: this creates an incentive for strong
>> parties and partisanship. See the Alaska 2022 Senate race,
>> where Democrats pulled off a favorite-betrayal in support of
>> Murkowski to avoid a center-squeeze.
>> 3. Is a counterstrategy available?
>> 4. How feasible is the strategy (does it involve many or few
>> voters)?
>> 5. How bad would the effects of the strategy be? Borda is bad
>> not just because it's often susceptible to strategy, but
>> because it gives turkeys a solid chance of winning.
>>
>> IRV is a good example of this. It's /usually/ not susceptible
>> to strategy (in the IAC model), but I think of it as one of
>> the most strategy-afflicted methods on this list. It's
>> vulnerable to some particularly-egregious strategies
>> (decapitation), ones that are complex or difficult to explain
>> (pushover), and many strategies don't have a simple defensive
>> counterstrategy available (like truncation).
>>
>> A low-probability but occasionally high-impact strategy might
>> be the worst of both worlds; voters get lulled into a false
>> sense of security by a few elections where strategy doesn't
>> matter, then suddenly find a candidate they dislike elected
>> because they failed to execute the appropriate defensive
>> strategy.
>>
>> ----
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>
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