[EM] optimal Condorcet truncation

[Russ Paielli]

Russ Paielli 6049awj02-at-sneakemail.com |EMlist| wrote:

> 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. OK, then, lets 
> determine when and how the voter should truncate rather than waste time 
> making a useless criterion out of it. Could it be that good voting 
> strategy in Condorcet involves truncation at the Approval cutoff point? 
> Now *that* would be interesting.

After thinking more about this proposition, I think the Approval formula 
(see http://ElectionMethods.org/Approval-formula.htm) applies to 
Condorcet voting also. The Approval formula simply says to approve any 
candidate that is above the expected value of the entire election. The 
same reasoning applies to Condorcet voting. The difference is that, in 
Condorcet, the voter ranks the approved candidates, and the unapproved 
candidates are not ranked at all.

By following that rule, the voter eliminates the chance that he will 
help a candidate below his threshold defeat one above his threshold. In 
other words, this truncation strategy will optimally mitigate the 
effects of the failure of Condorcet to pass SPC.

The hard part of applying the rule will probably be the estimation of 
the winning probabilities, but that is no different than for using it in 
Approval.

If this has been suggested before, please let me know where. Thanks.

--Russ