# [EM] CDTT,IRV

James Green-Armytage jarmyta at antioch-college.edu
Thu May 26 19:57:46 PDT 2005

```Hi Chris,
Here is the second part of my reply, which is on the subject of CDTT,IRV.
>
James:
>
>As far as I understand CDTT-IRV, the basic strategic vulnerability is
>that if the sincere IRV winner X differs from the sincere Condorcet winner
>Y, X>Y voters will have an incentive to bury-reverse Y.
Chris:
>
>CDTT,IRV  is less vulnerable to Burial  than Defeat-Dropper(Winning
>Votes)  or any other plain rankings method that meets
>Mutual Majority, Smith(Gross) and Clone Independence.
>Take this classic example:
>49: A   (sincere is A>B)
>24: B
>27: C>B
>One of the definitions of the CDTT is "the set of candidates that all
>have a majority strength beat-path to the candidates that have one to
>them."
>The CDTT here is BC. In the IRV count, B is first eliminated  and so C
>wins (an example of failing the Plurality criterion).
>The  A supporters  can do nothing to get A into the CDTT, but they can
>gain a result they prefer by voting sincerely.
>The B supporters can do theselves no harm by voting B>A if  they want to.
>Of course, unlike plain IRV, it fails Later-no-Help. The C supporters
>help C by ranking B.

I'm not sure that your example contradicts my general statement, because
I don't know who the sincere IRV winner was. Maybe A was the sincere IRV
winner, but all of the B voters truncated. Perhaps I could make my
statement more tautological as follows:
__If the sincere IRV winner I differs from the sincere Condorcet winner
C, and all C>I voters vote strict and sincere rankings, then the I>C
voters will have an opportunity to give I the victory by using a
bury-reverse strategy against C.__ (Is this always true? I'm not sure, but
I think so.)

Your example may be something of a special case in that B is such an
obvious CW that voters can plan their strategies around that premise. Most
ER-IRV and Condorcet methods probably work okay when the CW is blatantly
obvious to all voters.
This example may also something of a special case in that A doesn't have
a majority beat over anyone, which greatly limits the A voters' strategic
options. If A had a majority beat over C, it would be a different story.
>
>
>One possible political advantage  of  CDTT,IRV is that it can be sold as
>an improved form of IRV.  I  think that is better than jumping in front
>of the IRV
>movement and shouting "Go Back! IRV is evil!".

Yes, CDTT seems like one interesting way to bring IRV toward
Smith-efficiency. CWO-IRV is another, in my opinion.
>
CDTT,IRV looks like my July 26 UMID,IRV proposal.

Here's CDTT (from a 2003 draft of a Woodall paper? how did we get it?):
>union of all minimal nonempty sets of candidates such that no candidate
>in each set has a majority-strength pairwise loss to any candidate
>outside of the set
and here's UMID from 7/26/04:
...
>If there is a set such that no candidate within the set is
>majority-beaten by any candidate outside the set, then it's an
>inconclusively-dominated set. If it doesn't contain other
>inconclusively-dominated sets, then it's a minimal
>inconclusively-dominated set. So the union of minimal
>inconclusively-dominated sets consists of all the candidates who belong
>to a minimal inconclusively-dominated set.

That's the same, right?

my best,
James

```