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James,<br>
I have always regarded equal-rankings allowed IRV(fractional) as a small,
mostly irrelevant refinement of normal no equal-ranking<br>
(except for truncation) allowed IRV which would never be implemented because
it makes counting more difficult (especially if <br>
hand-counting paper ballots), and the demand for it from voters and parties
would be very small.<br>
So,like you, I was pleasantly surprised to see that this seemingly small
refinement is a suffiiciently big improvement on standard IRV<br>
for Mike Ossiopoff to rank it both above "Majority-Choice Approval" (MCA)
and Approval. According to Mike, it meets his <br>
"Weak Defensive Strategy Criterion" (WDSC). From electionmethods.org:<br>
<p><em>If a majority prefers one particular candidate to another, then they
should have a way of voting that will ensure that the other cannot win, without
any member of that majority reversing a preference for one candidate over
another.</em><br>
<br>
Judging by the example at Steve Eppley's site, it seems to meet his (similar)
"Non-Drastic Defense" criterion. I can't see or imagine any<br>
possible theoretical disadvantage ER-IRV(fractional) could have compared
to standard IRV, so (in light of the above) I rate it as <br>
unambiguusly better.<br>
<br>
The same cannot be said of ER-IRV(whole). Unlike standard IRV, it fails
the Symetric Completion criterion and the "No Zero-Information<br>
Strategy" standard. The voter with no idea of how others vote, who has a
sufficiently large gap in his/her ratings, now does better to <br>
insincerely rank all those candidates above the gap in equal-first place.
But that is far from the worst of it!<br>
<br>
Take this example of sincere preferences:<br>
45:Right>CentreRight>Left<br>
35:CentreRight>Right>Left<br>
20:Left>CentreRight>Left<br>
<br>
CentreRight is both the sincere CW and IRV winner.<br>
IRV is vulnerable to the "Push-over" strategy. This from EMR:<br>
<br>
</p>
<p><a name="push-over"></a><b>push-over</b> <br>
The strategy of ranking a weak alternative higher than one's preferred alternative,
which may be useful in a method that violates <a
href="http://condorcet.org/emr/defn.shtml#monotonicity">monotonicity</a>.</p>
<p>In the above example, some (but not too many) of the Right supporters
can use the Push-over strategy to make Right win:<br>
<br>
25:Right>CentreRight>Left<br>
20:Left>Right>CentreRight (these are Push-over strategising Right
supporters)<br>
35:CentreRight>Right>Left<br>
20:Left>CentreRight>Right<br>
</p>
<p>Now CentreRight has the lowest first-preference tally, and then Right
wins. The strategists had to be sure that Right had a pairwise<br>
win against Left, and that Right wouldn't be eliminated. It could be difficult
or risky to coordinate, because obviously if too many Right<br>
supporters vote that way, then Left will win .<br>
But look what happens when the method is ER-IRV(whole)! Now the Right supporters
have a vastly improved Pushover-like<br>
opportunity.<br>
<br>
45:Right=Left>CentreRight<br>
35:CentreRight>Right>Left<br>
20:Left>CentreRight>Right<br>
</p>
<p>First-preference tallies<br>
Right:45 CentreRight:35 Left:65<br>
</p>
<p>CentreRight has the lowest tally, and so is eliminated then Right wins.
<br>
This time no coordination was needed. As long as the Right suporters knew
that Right had more first-prefernces than CentreRight, and a<br>
pairwise win against Left, then each individual Right supporter got an increased
expectation by insincerely upranking Left from last to<br>
equal-first with no risk.<br>
This example wouldn't work if there was a "majority stopping rule" (because
then Left would be declared the winner on the first round),<br>
but if there was, then we would have an Approval-like method with lots of
insincere compression incentive, that I am sure would fail<br>
Clone Independence.<br>
In the example, with ER-IRV(fractional) the same strategy by the Right voters
would also succeed, but the strategists had less margin <br>
of error, and in general it is much easier and less risky with the whole
votes version. But contradicting what I wrote earlier, maybe it is a<br>
significant disadvantage of ER-IRV(fractional) versus plain IRV that Push-over
strategising is less risky and more tempting.<br>
In conclusion, ER-IRV(whole) is worse than standard IRV. ER-IRV(fractional)
may be better than plain IRV, but I don't like its<br>
chances of being introduced in practice. I would think that most voters wouln't
see much point in it, and election officials would hate it.<br>
<br>
Chris Benham<br>
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