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Re: Norman Petry and I (Ossipoff) recommended CSSD, but Schwartz Woodall is a better voting system for Debian

Hi Michael,

On Thu, May 09, 2013 at 04:11:58PM -0400, Michael Ossipoff wrote:
> [quote]
> However this seems quite a risky strategy by the B voters.  The
>  situation seems contrived and unlikely to arise in practice.
> [/quote]

> But what if the B voters know for a fact that the A voters are
> completely conscientious, responsible, and co-operative, and that the
> A voters are sure to rank B over C?

> Sure, I agree that there are a number of good reaons why the chicken
> dilemma needn't be a problem. But, even if it isn't a full-fledged
> problem, it remains a _nuisance_.

For me to consider this a nuisance, I would have to see that there is a
practical case where a rational group of voters might use this strategy to
sway the election in their direction.  Clearly, there are cases where CSSD
would reward strategic voting *if* a voting bloc had perfect knowledge of
how everyone else would vote.  But is that realistic?  In your original
scenario, the preferences are:

 99: A>B>>C
 2: B>A>>C
 100: C>>(A=B)

But how are the B voters to know this with certainty?  Even one voter
preferring A>C>B (or at least, voting that way) is sufficient to undermine
this strategy, because instead of stealing the vote for B, they're suddenly
throwing the vote to C, which is the outcome they strongly want to avoid:

  98: A>B
  1: A>C>B
  2: B
  100: C

C defeats A, 100>99; A defeats B, 99>2; C defeats B, 101>100; so C is the

The reason we care about these properties of voting systems is that we want
to avoid rewarding strategic voting.  I posit that a strategy that allows
for a margin of error of <1% in the attackers' understanding of how all
other voters will vote before it yields a pathological outcome instead is
not a very rewarding strategy at all.

All other things being equal, it would of course be better to address the
chicken dilemma.  However, you bear the burden of demonstrating that all
other things actually are equal.  The method you propose has been evaluated
with respect to a couple of important criteria (the Mutual Majority
Criterion and the Condorcet Criterion); but what about other criteria that
CSSD satisfies?  There are lots of criteria that are interesting to students
of voting, and it's well known that some of them are mutually exclusive;
before making any changes to our voting system, we should understand the
consequences fully, not just with regards to a couple of handpicked criteria
that are superficially the most important.

Where can we find public, third-party review and analysis of the method you
propose (which seems to be a hybrid of other methods - so I'm not sure if it
can properly be called "Schwartz Woodall" or not?)?

Since the voting algorithm is enshrined in the Debian constitution, the cost
of changing it is high; the burden of proof when arguing for a change is
therefore high as well.

Steve Langasek                   Give me a lever long enough and a Free OS
Debian Developer                   to set it on, and I can move the world.
Ubuntu Developer                                    http://www.debian.org/
slangasek@ubuntu.com                                     vorlon@debian.org

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