[EM] IRV et al v. EPR
km_elmet at t-online.de
Mon Jul 16 12:21:50 PDT 2018
On 2018-07-16 20:09, Jameson Quinn wrote:
> This system as described is very close to being an excellent
> "Proportional Majority Judgment" method, but has one key flaw. When a
> candidate is elected with more than 1 quota of support, all of their
> supporting ballots are marked as used. To give a proportional method and
> to minimize strategic incentives, only 1 quota of supporting ballots
> should be marked as used. This could be done through some ordering
> criterion (highest support for winner/lowest support for others), by
> proportionally reweighting ballots, or by using up randomly-chosen
> ballots; the differences between these three options would be relatively
> This reflects the basic way to transform any single-winner method into a
> proportional multi-winner method: find single winners sequentially, and
> then for each of those winners, "use up" the one quota of ballots that
> "contributed most" to making that candidate the winner. There's room for
> judgment calls in defining "contributed most", but other than that this
> is a general template that IMO gives an optimal combination of good and
> practical from methods as varied as IRV (which becomes STV), MJ, STAR,
> Score, approval, Condorcet... in short, almost any single-winner method.
Most of those multiwinner methods have unweighted winners. In EPR (as I
understand it), each winner has a different weight in the assembly, and
thus instead of discarding just a quota and then redistributing the
surplus, it's possible to assign more than a quota to a single winner,
who benefits from this "supermajority" by an increased weight.
To use SNTV as an example, suppose you have a Plurality election of the
and three to elect. If you use ordinary SNTV, then the voters who voted
for A wasted their votes, since they voted in excess of what was
required to have A win. However, with weighted voting, it's a simple
matter of letting A have a weight of 47.6% of the total vote, B have a
weight of 38.1% and so on.
Suppose e.g. that the A- and B-voters both had D as their second
preference. Unweighted, the wasted votes for A and B deprived D of his
victory, as in SNTV, some fractions of the A- and B-voters could have
strategically voted for D instead to get him above C's count, and that's
what a better method with surplus redistribution would have done anyway.
In a weighted method, the A and B-voters get compensated for C being
elected, in the form of A and B having a greater share of power; and
this is preferable from the point of view of A- and B-voters because
they get to contribute directly to the power of their first choice
candidates instead of having that power go to their second choice.
With all that said, there's another argument that could be made in favor
of doing surplus redistribution, which is that under tactical
nomination, you could get something analogous to surplus redistribution
anyway. If the A-voters consider D to be close to A, then some of them
could vote for D, after which the fixed council size would push C off.
The benefit of reducing C's strength to zero could then make up for A's
relative power being reduced in the council.
A stronger strategy would involve cloning A into A1 and A2 and, instead
of redistributing votes to D, distributing them between A1 and A2. That
produces a more party list-like outcome -- but that strategy would also
be possible under an unweighted multiwinner system.
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