James G-A,
You wrote (Thur.May 26):
Chris, you write:
my example regarding ER-IRV(whole).
45:Right=Left>CentreRight
35:CentreRight>Right>Left
20:Left>CentreRight>Right
First-preference tallies
Right:45 CentreRight:35 Left:65
CentreRight has the lowest tally, and so is eliminated then Right wins.
This time no coordination was needed. As long as the Right suporters knew
that Right had more first-prefernces than CentreRight, and a
pairwise win against Left, then each individual Right supporter got an
increased expectation by insincerely upranking Left from last to
equal-first with no risk.
This would also work if the numbers 45/35/20 were replaced with 49/48/3.
I suggest the right numbers in your "paradoxical" row should be IRV1,
ER-IRV(fractional)2,
ER-IRV(whole) 5!
You may be right about this, but let's discuss a bit more. ER-IRV(whole)
is a good topic that doesn't get explored enough.
It seems that the L voters can set things right by voting L=CR>R. Or, if
some of the CR voters get mad at the R voters and truncate, L wins. It
seems that the CR voters have little reason to rank R second, since they
should know that R doesn't need their help to beat L. If there is even a
hint of R voters betraying CR, I would expect the CR voters to bullet
vote.
In some real-life elections, for some voters there are more important
issues than the strategising (or non-strategising) of the supporters of
rival candidates.
In rankings election the desirable norm should be that voters give a
full sincere ranking. If voters aren't interested in strategy, and they
have a preference order they wish to express,
then why shouldn't they give it?
Even if we grant the assumption that the voters are alert to strategy
threats and are primed to "retaliate", there is no reason why there
should be "even a hint of R voters betraying CR".
The strategy is low-risk and doesn't require any coordination (unlike
Pushover in normal IRV), so individual R voters can do it on their own
initiative.
In Australian parliamentary elections, most (or at least very many)
voters decide which party they are going to vote for (usually their
sincere favourite) and then just take that party's
"how-to-vote card" from one of the volunteers at the polling station
and copy it.
So in the above scenario, the R's "betrayal" of CR could just take the
form of springing these "surprising" how-to-vote cards on polling day.
("Gee, there must have been a mix-up
at the printers!")
Of course in Australian elections, voting is compulsory and in most IRV
elections so is full ranking. But sometimes there is another reason why
voters might want to rank fully.
A few years ago, there quickly arose a new far-right party that was
widely regarded as racist; and they had a big profile in the media. I
refer to the rise in Australia of the "One Nation"
party. There was a campaign for voters to "put One Nation last!" to
make a statement rejecting racism.
I meant a vulnerability score of 5 (now 6) to indicate a really critical
vulnerability, like the compromising incentive in plurality or the burying
vulnerability in margins. Do you really think that the paradoxical
vulnerability in ER-IRV(whole) is at that level?
I assumed that the scores were for comparing the vulnerability of
methods along the rows, not comparing how big a problem the different
vulnerabilities are relative to each other,
down the columns (within each method).
So I suggested the score of 5/5 because I can't even envisage another
plausible method being more vulnerable to this "paradoxical" strategy.
No, I don't think the problem is as
bad as the Compromise incentive in FPP; but I do think its bad enough to
disqualify ER-IRV(Whole) as a serious proposal.
More importantly (since I'm not even sure that I will cover ER-IRV(whole)
in my little chart), do you think that the disadvantages of ER-IRV(whole)
due to this issue outweigh its advantages over ER-IRV(fractional) in terms
of reduced compromising-reversal incentive?
Yes! I regard ER-IRV(whole) as much worse than plain IRV and that
ER-IRV(fractional) is probably slightly worse than plain IRV (and
certainy not worth the extra complexity).
Reducing incentive to compromise-reverse just by swapping it for
incentive to compromise-compress only seems to have any point if we
assume that the voters notice these incentives
and are interested in strategising.
As I put it in a message to Forest, I think the "official assumption"
should be that they aren't. (And of course the compromise-reversal
incentives in plain IRV are usually much less powerful than they are in
FPP).
Chris Benham
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