[EM] MOP-F2 / Later-no-Help

[C.Benham]

On 9/07/2019 3:16 pm, Juho Laatu wrote:

> Later-no-Help is not one of my favourites either. I would have some sympathy towards "Later-yes-Help". That is because Later-no-Help seems to tell the voters that it is ok to truncate and not give their sincere rankings, while "Later-yes-Help" says that voters would be better off if they would tell the method all their preferences.
Since the method is only interested in finding a single-winner, how is 
it fair that some voters would be "better off" (than other voters) 
because they
"tell the method all their preferences"?

Getting back to your quibbles about the Plurality criterion (which you 
say you "don't like"), when I first started thinking about single-winner 
voting
methods, I (living in IRV land) thought it was "obviously" highly 
desirable that a ballot that has an irrelevant weak candidate ranked 
above some
other candidates should have exactly the same effect (and so probably 
"significance" in the view of criteria) as a ballot that ignores (or 
just lower
ranks) that weak candidate but ranks all the other candidates in the 
same order (so has a different candidate ranked below no others).

However on becoming better acquainted with all the problems and issues, 
I now think property is far less important.

We get a higher "social utility", more legitimate-looking winner if the 
voters are discouraged from expressing their very weak, perhaps light-minded
ill-informed preferences that the method is unable to distinguish from 
their serious strong preferences.

Even if the method allows the voter to do nothing except rank the 
candidates, we nonetheless logically know which are the voters strongest 
most
serious pairwise preferences: their preference for the candidate/s they 
vote below no others over the candidate/s they vote above no others.
If I had coined something like the Plurality criterion to take account 
of ballots that allow equal-top ranking and not have any possible confusion
or ambiguity about what a "vote" is, it would say something like "If A 
is voted above B and below no other candidate on more ballots than B is
voted above any candidate, then B can't win."

But, based on your vague negativity regarding the Plurality criterion 
and some of your other statements, apparently you have it as a 
"principle" that
this information should be ignored (at least in methods that don't allow 
the voters to give any explicit approval cutoff).

We infer from your "some sympathy for later-yes-help" that you like 
Later-no-Harm.  Both that and Later-no-Help are incompatible Condorcet,
which is why when attacking Margins (while promoting some other 
Condorcet methods) I refer to "egregious" failure of Later-no-Help (going
along with failure of Plurality).

> This means that a "Later-Irrelevant-Alternatives-no-Help" could be a better criterion than Later-no-Help.

Juho, How (for this purpose) would you define an "Irrelevant Alternative"?

Chris Benham

On 9/07/2019 3:16 pm, Juho Laatu wrote:
>> On 08 Jul 2019, at 17:36, C.Benham <cbenham at adam.com.au> wrote:
>>> Margins provide good results with sincere votes, so why not use margins...
>> I don't see how egregious failures of the Plurality  and Later-no-Help (and even Non-Drastic Defense) criteria constitute "good results"
>> irrespective of whether the votes are "sincere" or not.
> Later-no-Help is not one of my favourites either. I would have some sympathy towards "Later-yes-Help". That is because Later-no-Help seems to tell the voters that it is ok to truncate and not give their sincere rankings, while "Later-yes-Help" says that voters would be better off if they would tell the method all their preferences.
>
> On the other hand, it would be good if voters are not punished too much if they truncate in an election with hundreds of candidates. Truncation of candidates that have no chances to win should be harmless. This means that a "Later-Irrelevant-Alternatives-no-Help" could be a better criterion than Later-no-Help. I would at least strongly encourage voters to rank all (hopefully not too many) potential winners (except the last one, whose position can be made clear already by ranking all the others).
>
>
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