[EM] Re: Cheering for simplicity/Orphan

[Rob LeGrand]

John B. Hodges wrote:
> CPO-STV is an awesome multiseat method, conceptually. I'm wondering
> if there is a computationally efficient way of arriving at the same
> "ideal" ensemble. My "For Dummies" guess is that the ideal ensemble
> will never include a Condorcet loser and will always include a
> Condorcet-winner if one exists. STV with Rob's "orphan" elimination
> rule would (I guess) be sufficient to do that much.

I'm not sure I agree that the ideal ensemble should always include a
Condorcet winner and exclude a Condorcet loser.  For example, if we're
electing two winners from three candidates and the preferences are

45:Reagan>Anderson>Carter
20:Anderson>Carter>Reagan
35:Carter>Anderson>Reagan

I would argue that the outcome {Reagan, Carter} is the most proportionally
representative.  In a sense, the Reagan-first voters' preference for
Anderson over Carter *should* be ignored since they already got their first
choice in Reagan.  (Would anyone disagree with that?  Is there a good
argument for preferring the outcome {Carter, Anderson}?)  So this "ideal
ensemble" includes the Condorcet loser (Reagan) and excludes the Condorcet
winner (Anderson).

The moral here is that single-winner and multiwinner elections are quite
different.  While the best single-winner methods can concentrate on
limiting insincere voting without worrying about proportionality, the best
multiwinner methods must strike a balance between striving for good
proportionality and discouraging insincerity.

I think of a spectrum with single-winner systems at one end and something
like Direct Representation (http://www.directrep.org/) at the other. 
Strewn in the middle are n-winner systems.  Somewhat paradoxically, "vote
for any number of candidates" (Approval) is best for the single-winner case
and "vote for only one candidate" (SNTV) becomes best for the n-winner case
as n increases.   Approval is great at removing insincerity from strategy
but gives poor proportionality (which is irrelevant in the single-winner
case), while SNTV often encourages insincerity (though less and less as n
increases) but gives good proportionality (which improves as n increases). 
So it's not obvious to me that the best multiwinner systems reduce in the
single-winner case to the best single-winner systems.

=====
Rob LeGrand, psephologist
rob at approvalvoting.org
Citizens for Approval Voting
http://www.approvalvoting.org/

__________________________________
Do you Yahoo!?
Yahoo! SiteBuilder - Free, easy-to-use web site design software
http://sitebuilder.yahoo.com