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

[Juho Laatu]

> On 12 Jul 2019, at 19:15, C.Benham <cbenham at adam.com.au> wrote:
> 
> 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"?

By ranking more candidates they would increase the likelihood of one of their favourites (or acceptable candidates) winning.

> 
> Getting back to your quibbles about the Plurality criterion (which you say you "don't like"),

Yes, because of treating different positions in the ranking order differently is not really suited for _pure_ ranking based methods.

> 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).

The "philosophy" of IRV is to put quite a lot of weight on first preferences in the counting process. The plain ranking approach where only relative preferences count may be better for most elections (especially compromise oriented ones). Both approaches are acceptable to me, but pure ranked methods are in some sense quite elegant and yield balanced results. It depends also on what one really wants. Would it be wise to elect a candidate with lots of first preference support, or a candidate with lots of support in pairwise comparisons against other candidates.

> 
> 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.

It may be good to rank also one's favourite weak candidates for the reason that those candidates might get some support in this election, and more in the next election. The election may thus serve also as means to present new candidates to the electorate.

From voter's point of view it also makes sense to rank some candidates that the voter thinks are not serious contenders. That makes sense since the voter can not know beforehand how much support each candidate will get. Some of the candidates might well be more popular than he thinks.

From this point of view, maybe one should rank all candidates that could, under favourable circumstances, be serious contenders in this election or in the foreseeable next elections. If one ranks candidates that do not belong to these categories, maybe then we are talking about protest votes, or maybe also votes to one's friends whom one wants to make happy although they are not likely to ever win, or maybe about voters who are not good at estimating who is likely to win. These are also not bad things to do, and the election method should not get confused if many people vote this way.

> 
> 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.

Yes it is obvious that the the preference of the best candidate against the worst candidate is the strongest preference. In ranked methods we however typically have to satisfy with putting the same weight to all preferences (=1). Otherwise we might get into the problems of Borda and Range (= strategic exaggeration).

> 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).

Yes, I think that if the method puts some specific emphasis on some position in the ballot, it would be good if voters would be fully aware of that.

I tend to think that if we talk about pure rankings, then it should have no meaning if some candidates have been ranked tied first or tied last or one but last. Only the relative rankings should count. One could try to derive the true preference strengths e.g. from the fact that preference strength between the first and last ranked candidates must be the strongest preference strengths. But that is difficult. Range is one method that tries to achieve this, but that approach is not problem free. James Green-Armytage's CWP makes quite good use of different preference strengths. Also MOP-F2 tries to do the same by allowing voters to tell which preference relations should be considered weak if there is a (sincere or strategic) loop.

> 
> We infer from your "some sympathy for later-yes-help" that you like Later-no-Harm.  

I would like also "Nowhere-no-Harm". :-) But yes, in principle Later-no-Harm is a good thing. I'm however willing to accept some compromises when it comes to meeting all the positive sounding criteria 100%. Sometimes there are also conflicting interests that might be even more important. There are many criteria that I think "should be respected practically always but not necessarily in all situations". One example is the Smith set criterion, that will and should be true in practice in all elections, but there are also special (very uncommon) situations where it is easy to say that one should not follow that rule.

> 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).

I see Later-no-Help criticism as referring to situations where the later preferences somehow create a loop that might help one's favourite candidate. Not a big problem. If that loop is sincere, then the end result is probably what it should be. If that loop is strategic, we have a slightly bigger problem in our hands, but the probability of such scenarios ever materialising into successful strategic attacks in typical public elections is low. Do you think the problems are more serious that this (in real life elections)?

> 
>> 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"?

What I meant here is candidates that have no chance of winning, and can also not play any role in determining the winner, even if some voters use them strategically. It is good that at some point the voter can stop ranking more candidates (if the number of candidates is high), and trust that only the already given rankings are relevant in determining the winner. I think it would be however important to rank at least all the potential winners (except maybe the last one).

Juho


> 
> 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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