[EM] Questions about Majority-Beat vs Plurality-Beat Condorcet

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On Sat, 4 Apr 2026 00:12:16 +0000 (UTC)
Kevin Venzke <stepjak at yahoo.fr> wrote:

> Hi Gustav,
> 
> I'm not familiar with some of your terminology, but since I came up with ICA and the
> tied-at-the-top rule, and am usually the one who tries to prove/disprove that
> methods satisfy the weak FBC (which you call AFB), I will try to respond to some of
> your questions.

Thanks for your answer

I suppose I should clarify some.
A candidate, say Alice, Plurality-Beats another candidate, say Bob,
in a pairwise matchup if the number of votes ranks Alice strictly above Bob
is strictly greater then the number of votes ranking Bob strictly above Alice,
v(Alice>Bob) > v(Bob>Alice).
For Alice to Majority-Beat Bob the requirement changes to the number
of votes ranking Alice strictly above Bob to be strictly greater then
half the total number of votes, which assuming compleat rank orders
are provided becomes v(Alice>Bob) > v(Bob>Alice) + v(Alice=Bob)
or simply v(Alice>Bob) > v()/2.
(I will stick with using the most often used minimal majority,
that is half the total voters, as the majoity threashold.)
A candidate is a Plurality-Beat Condorcet winner if the Plurality-Beat
every other candidate in a pairwise matchup (PB-Condorcet winner),
but for a candidate to be a Majority-Beat Condorcet winner (MB-Condorcet winner)
they need to Majority-Beat every other candidate in a pairwise matchup.

At this point it follows that being a MB-Condorcet winner implies also being
a PB-Condorcet winner since Majority-Beating someone imples also
Plurality-beating the same someone,
but it is possible to Plurality-Beat without Majority-Beating.

As for the criterias, to always elect a PB-Condorcet winner of the ballot preferences
if one exist becomes the PB-Condorcet criteria, exactly the same as the
Condorcet criteria on electowiki and most other places.
To elect a MB-Condorcet winner of the ballot preferences if one exist
becomes the MB-Condorcet Criteria, which does not appear to have any
common name, which is why is stuck with this prefixing indicating beat type.

Since MB-Condorcet winner also are PB-Condorcet winner,
satisfying PB-Condorcet criterial also satisfy MB-Condorcet criteria,
but since a candidate can be a PB-Condorcet winner without being a
MB-Condorcet winner, it is possible for a voting system to fail the
PB-Condorcet criteria but still satisfy the MB-Condorcet criteria.
I noticed both the regular ICA-T&T ( the regular one, T&T for tied and toped)
and the other system on the same page,
which I will here call ICA-T&A (for tied and approved),
both satisfy the MB-Condorcet criteria while failing PB-Condorcet.
ICA-T&T mixes Plurality-Beating and Majority-Beating based on first preferences while
ICA-T&A sticks only to Majority-Beating when testing for a "condorcet winner".


> Le vendredi 3 avril 2026 à 17:12:57 UTC−5, Gustav Thorzen via Election-Methods <election-methods at lists.electorama.com> a écrit :
> > So I have been trying to learn about voting theory on my own for a while,
> > but there are some things I am still not sure about,
> > especially when it comes to Majority-Beat (MB) vs Plurality-Beat (PB) Condorcet,
> > where the usual criteria appear to be implicitly assuming PB for pairwise matchups.
> > 
> > The tied at the top rule used in Improved Condorcet Approval (ICA)
> > allows the system to pass Avoids Favorite Betrayal (AFB)
> > but makes it fail PB-Condorcet while retaining MB-Condorcet.
> > I found it interesting that MB-Condorcet and AFB is compatible,
> > while PB-Condorcet and AFB is not,
> > but does the other method mentioned on the ICA wiki page,
> > which appears to be MB-Condorcet//Approval,
> > also satisfy AFB since it is not mentioned explicitly,
> > and is the other method equivalent to MB-Condorcet//Approval?
> 
> From memory, the other method was MinMax(WV). I suppose it's possible this method
> has obtained another name of "MB-Condorcet," though I'm not sure why.
> 
> The other method is not equivalent to MinMax(WV)//Approval because MinMax(WV) is a
> true Condorcet method, and that isn't compatible with weak FBC.

So the other method, ICA-T&A is not a "true Condorcet method" since it does not satisfy
the PB-Condorcet criteria, it is not obvious if it satisfy weak FBC
(which I call AFB since I found "avoids" lead to less confusion with people I talked to,
I will switch if that is prefered on this list).

> > It is also not mentioned if any of the satisfy Participation leading to the next questions.
> 
> Definitely not. Very few methods satisfy Participation, certainly not ones that
> resemble Condorcet. The most complicated Participation methods are DAC and DSC.

Yeah, Participation is clearly a rare and difficult one.
I was thinking since MB-Condorcet turned out to be compatible with AFB,
maybe it would also be compatible with Participation,
and if any of the ICA:s are compatible with both I would have a good
starting point for figuring out what it takes of a system to satisfy all three.
Again (my bad for being unclear about PB vs MB) since the ICA:s only satisfy
MB-Condorcet criteria while failing PB-Condorcet criteria,
it is unclear if they satisfy or fail participation since the usual
PB-Condorcet criteria imply failing Participation is not relevant here.

> > While PB-Condorcet, PB-Smith, and, PB-ISDA, each implying the previous ones,
> > are all incompatible with AFB, Participation, Later-No-Help/Harm (LN-Help/Harm),
> > and becomes vulnerable to Dark Horse + 3 Rivals (DH3R) unless the fail Reversal Symmetry,
> > the MB-Condorcet is compatible with AFB, so in addition to that,
> > are the MB-Condorcet, MB-Smith, and MB-ISDA compatible with and of these criteria
> > and/or can satisfy Reversal Symmetry without vulnerability to DH3R?
> > (No claim whether or not the trade of combining MB-Smith with LN-Help+Harm is worthwhile.)
> 
> I'm very confused, but if MB-Condorcet is a criterion that implies Condorcet, then
> it is not compatible with AFB.

I entierly agree with the above logic, but with it clarified how it is the
PB-Condorcet criteria instead implying the MB-Condorcet criteria,
it should hopefully be more understandable why I am curious if
the Majority-Beat versions of Condorcet+Smith+ISDA are compatible
with any of AFB, Participation, Later-No-Help/Harm.
I would very much lite the intuitive concepet of condorcet winner
earning a win be compatible with those, and the "failure" of
PB-Condorcet criteria is due to the Plurality-Beat rather then the Condorcet.

> I an not sure of a good way to demonstrate invulnerability to DH+3R short of just
> requiring that burial never works, or not using lower preferences... I have never
> been clear on the question of to what extent we think voters in this scenario are
> using good strategy or possess good information. If the voters aren't rational then
> I don't think we should expect there is a solution for us in simply preventing
> burial from helping anyone.
> 
> > Assuming the system is required to be fully deterministic and voter/candidate symmetric,
> > so much so that the possibility of a no-winner outcome is assumed acceptable,
> > leaving us with a "at most 1 winner system".
> > Since Mutual Majority is incompatible with the above assumptions,
> > the earlier impossibility theorem of 3 out of the 4 of Monotonicity, Mutual Majority, LN-Help, LN-Harm
> > have been reduced to 3 of the 3 Monotonicity+LN-Help+Harm,
> > would it be desirable to satisfy all 3 at the same time?
> 
> Would it be desirable, certainly yes from certain perspectives. But you're very
> limited. You can satisfy it with FPTP (performed on rank ballots), for instance.

I never understood how FPTP can be considered to satisfy LN-Help+Harm when
those criteria are only defined for rank-orders rather then the name exactly one candidate
ballots FPTP uses. (I know we can create a system using rank-order ballot looking only
at first preferences to get a different system with the same outcome, but then
that is by definition not FPTP but a different system.)
Though that is probably irelevant to the discussion.
Thanks for the input on the desirablility of Monotinicity+LN-Help+Harm.
Personally I think the knowledge of how to create system satisfying thoose three
criteria would be desirable even if would reject those for Mutual Majority.
 
> > We would also loose MB-Smith and MB-ISDA since they are defined as a member of the set
> > must win no matter what, unless we redefine them to be candidates not in the set cannot win.
> > With the following change would it also be desirable to satisfy
> > MB-ISDA+AFB+Participation+Monotonicity+LN-Help+Harm if possible
> > if we ever found ourselves stuck with the requirement to be fully deterministic?
> 
> Would be highly desirable, but I don't think you're going to get ISDA at the same
> time as AFB or Participation. AFB offers a little more room to maneuver, but I've
> still never seen that it's possible to do anything where you e.g. let a candidate
> inherit some status through an indirect beatpath to another candidate.

And here I had the impression the answer would be the opposite.
If it turns out the "other candidates won't win" modification allows
compatability when randomness is allowed by intoducing a "no winner outcome"
on the ballots, would it still be desirable?
I doubt it would be possible while keeping outcome symmetry,
but maybe it still is under only candidate symmetry ("no winner" not a candidate).

> > The no-winner outcome appears to be found extremely unacceptable to the
> > point full determinism is thrown out without thought to simply to prevent it,
> > so I have been unable to find opinions on this scenario.
> 
> Well, in my view the possibility of a no-winner outcome means that we don't know how
> to apply our criteria anymore. A criterion like LNHarm is supposed to guarantee that
> a voter won't hurt themselves by providing additional info. If they provide the info
> and cause the result to be (or no longer be) "no-winner," what does that mean wrt
> the premise of the criterion?

Yes, most critera about which candidate wins have to be redefined for
fully deterministc system since they otherwise auto-fail.

> Even if we choose an answer to that question, I really doubt this will unlock some
> valuable criterion compatibilities for us.

MB-Condorcet potentially compatible with Participatin+LN-Help+Harm not a big deal?
It would mean that it is the Plurality-Beating rather the the pairwise winner concept
that is the problem, which I think the following example should make clear:

The candidate A, B, and C.
We have some number of votes where A > B where C is not listed,
and some other (possibly equal) number of votes where B > A and C is not listed,
and 0 or more number of votes where A = B and C is is not listed,
and exactly 1 vote where C > A and C > B (relative order of A and B is irelevant).

In this case C becomes the condorcet winner, a PB-Condorcet winner,
no majority required, no questing if A = B votes want to prevent either from
winning for are indifferent, only by voting on themself, or convincing a signle person.
C is not a MB-Condorcet winner.

Outside of this list and electowiki, this is almost always what the condorcet criterion requires,
that is the use of the PB-Condorcet criteria, in my experience.
Yes the example is trivially prevented by adding a counting rule
to append all unlisted candidates eaually to bottom rank,
to differentiate the concept of pariwise winner from Minority vs Majority-Beating.

Being able to show compatability between an "all pairwise matchup winner" criteria
and any of the criteria the PB-Condorcet criteria is incompatible with
looks to me like something of great value.

Hopefully this clears up any confusion.
Gustav