[EM] Truncation (was re: Defeat Strength)

[Kristofer Munsterhjelm]

On 10.09.2022 15:25, James Gilmour wrote:
> I think there is a lot of misunderstanding in this discussion under
> the Defeat Strength heading, now Truncation.  I suspect it may arise
> from a confusion of "IRV" with "preferential voting".  IRV = Single
> Transferable Vote applied to a single-winner election (= Alternative
> Vote in the UK) uses preferential voting, but there are several
> (many?) other ways of counting the votes recorded on ballot papers
> marked with voters' preferences.  You could use a Borda count, or a
> Condorcet count, or several different 'social choice' methods.  All
> have different implications for the "meaning" of preferences and the
> "meaning" of truncation (i.e. where the voter has not marked a
> preference against one or more candidates).

I understood Colin's response to be about what methods should do in 
general, similar to say, a criterion definition. The logic of such a 
general argument is something akin to:

- Suppose that a voter faces some particular situation.
- Then it's reasonable that the method should interpret the ballot or 
the voter's response in a way that's consistent with some property.
- Method X does or does not do this.
- Therefore this part about X is good/not so good.

So Colin is saying that it's reasonable that a method should give a 
voter an option to, in effect, mark a number of candidates as unknown, 
so that if the method has to decide between these candidates, it will 
not rely on that voter's ballot.

Such arguments are unconcerned with the algorithm that implements any 
particular method.

> What we are not at liberty to do is to take some aspect of the
> imputed philosophy of these other methods and say that it applies to
> IRV, or that it should apply to IRV.

We could check whether IRV does or doesn't pass certain criteria, 
though. E.g. I can say that Condorcet compliance is a reasonable 
extension of majority rule and adding a Condorcet preround never makes a 
method more strategically susceptible (for certain definitions thereof), 
and that this is a point against IRV. Or you could say that IRV passes 
both LNHs which very few other methods can, and that's a point in its favor.

As long as we have some notion of whether some philosophy or criterion 
is desirable or not, we can indeed check if IRV follows it. Now it may 
be that the designers of STV never intended it to follow that philosophy 
or pass that criterion, but that's not really relevant when determining 
whether a method is fit for purpose according to one's own standards, or 
those of some electorate.

-km