[EM] Re: optimal Condorcet truncation

[Chris Benham]

Russ,
You wrote (Sat.Feb.26):
"A more useful criterion is the normal (as opposed to Mike-style)
criterion taken from Blake Cretney's website:

Name: Secret Preferences Criterion: SPC
Application: Ranked ballots
Definition:
If alternative X wins, and some of the ballots are modified in their
rankings below X, X must still win.

Condorcet does not pass this criterion, which tells us that voters have
incentive to truncate in some cases if not routinely."

Woodall splits this somewhat oddly-named criterion into two fairly 
self-explanatory others:

"Later-no-Harm: adding a later preference to a ballot should not harm 
any candidate already listed", and
"Later-no-Help: adding a later preference to a ballot should not help 
any candidate already listed".

Condorcet passes neither of these, but your conclusion  only applies to 
Later-no-Harm.
In WV Condorcet (BP/RP/MM/River), the two LNHs are not in balance 
(adding a later preference is more likely
to help than harm an already listed candidate)  so that in the 
zero-information case there is a random-fill incentive.

As Kevin Venzke just more-or-less pointed out, the right 
zero-information strategy in WV is to equal-rank the candidates
 above some ("the") approval cutoff point and to strictly rank 
(random-filling if necessary) all the candidates below it.
Where is the cutoff point?  My interpretation of  an April 8,2000  Mike 
Ossipoff post on this question is that in the
3-candidate zero-information case if  your private rating of  your 
sincere first preference is 100 and your private rating of
your sincerely least preferred candidate is 0 and your private rating of 
your second preference is above 75; then you should
vote your top-two in equal-first place.

http://lists.electorama.com/pipermail/election-methods-electorama.com/2000-April/003903.html

If the electorate is polarised into two hostile factions, and the voters 
all follow this strategy; that means that which of the
winning faction's candidates actually wins will be decided by the voters 
who supported the other faction, and they might
have an incentive to pick the "worst" of them so that their faction will 
have a better chance next time.


Rob Le Grand wrote (Th.Feb.24, 05):

"Briefly, I prefer margins over
winning-votes for the following reasons.

1.  Zero-info strategy is fully-ranked sincerity under margins, but
usually not under winning-votes.  (It's often to a voter's
advantage to vote many tied ranks near the top.)
2.  Margins is more intuitive and easier to explain to most people.
3.  I haven't seen that winning-votes truly offers any valuable
strategic guarantees over margins."

Regarding  the first point, "fully-ranked sincerity" is arguably not a 
"strategy", so I like to refer to this property as meeting what
I call  the "No Zero-Information Strategy" (NZIS)  criterion. 
In WV Condorcet (MM etc.)  I dislike the ZI  "compress at or near the 
top" strategy, and  without absolute Later-no-harm  I
dislike a random-fill incentive.

I'll post something more constructive in a later post.

Chris Benham