[EM] Plurality-order principle

[Kevin Venzke]

Tom,

--- Tom Ruen <tomruen at itascacg.com> wrote:
> I see there's two major conflicting principles that we want in a single 
> winner election winner:
> 1. Plurality Principle - we want the winner who is can beat all others in 
> the full set of candidates. (Intensity of support)
> 2. Condorcet Principle - we want a winner who can beat all other 
> head-to-head (Breadth of support)

I won't criticize your principles, but I think the two together would be
hard to marry. #1 seems to be about the method's ability to focus voters
on a small set of candidates, while #2, in my mind, is about the ability
of voters to vote, and candidates to enter, without having to think so
much about strategy.

> Runoffs (with a majority winner constraint) are a sort of compromise between 
> these two principles. In the final round you know that the chosen winner 
> will also be the Condorcet winner for that final set of surviving 
> candidates. That's a nice little piece of knowledge.

I don't understand this. How do we know the chosen winner is the CW for
the final set? Are there only two candidates there?

> For me this is the #1 feature of IRV which I think will cause politicians to 
> reject IRV. If I'm a candidate and I reach first or second in the first 
> round, I EXPECT I should NEVER have to face elimination except to be 
> defeated by a united majority against me. However IRV can have this result 
> in a race where second and third place are close. (Like A=39%, B=30%, C=28%, 
> D=3%)

That's kind of an interesting principle. How about this: The first-round
front-runner can never be eliminated. Other than that, IRV rules are used.

If you privilege both of the first round's top two, you could end up
with a "right vs. other right" scenario, where the left candidates don't
have a chance, even if they have a majority. And that wouldn't really be
much better than plurality, I don't think.
 
> I call this general idea as: Respecting Plurality Order. This is what 
> candidates see in their expectations for success. (I accept that "plurality 
> order" is not a stable measure since it can change if you change the 
> original set of candidates, but there's nothing to be done about this except 
> completely abandon plurality counts as valuable.)
> 
> I've never seen this "respect plurality order" idea expressed in this way, 
> but I find it useful - most of all to reject bottom-up IRV.

I find it to be kind of a strange idea, since you don't seem to like
the plurality method. If you like the fact that plurality encourages
large first-preference scores, what could be so bad about it?

What defense do you use when a method elects the plurality runner-up?

> I often wonder that without any 
> spoiler effect, that candidates would too easily multiple beyond the ability 
> of voter to make an informed choice.

I doubt this. I think voters will create their own spoiler effect, no
matter the method, by not submitting complete rankings (particularly as
the number of candidates increases). Also, I tend to think that whoever
it is funding campaigns would feel pressure to get behind the most viable
candidates. I could be wrong about that.

> For me this also suggests a "progress" of methods from existing plurality 
> towards "spoiler free" Condorcet. And until the Condorcet pairwise process 
> becomes accepted legally as a valid counting method within "one person, one 
> vote", I see the best we can do is a Top-two Runoff. (OR Top-two Instant 
> Runoff)

I think we should expect the non-instant runoff to multiply the number
of candidates quite a bit. I was reading a book recently where the authors
argued for plurality and against two-stage runoff for the reason that
candidates don't have enough incentive to compromise before the first
election.

Kevin Venzke


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