Truncation Resistance #2 criterion (was Re: First Choices tiebre

[Hugh Tobin]

Steve's proposed consensus censorship raises the questions: (1) Why is
truncation resistance more important that resistance to order-reversal?
(I note that Mike defined both as "drastic" when used as defensive
strategy; in either case the strategic voter risks giving victory to his
least favorite, as Steve has explained). (2) Why it it critical to
prevent a putatively organized and devious group of plurality voters
from achieving through truncation what they could also achieve by
insincerely dividing their second choices -- or what half of them could
achieve by reversal, if one assumes the true second choice of all of
them is the middle candidate?

I submit that a system which reaches a different result from Steve's
second example if the "A" voters are divided into 23AB and 23AC creates
a clear incentive to insincere voting.

Given that incentives to strategic voting exist in all systems, it seems
the burden is on Steve to show why the incentive that exists if A wins
in example 2 is presumptively worse than those that may exist in a
universe of unspecified systems.

-- Hugh Tobin

Steve Eppley wrote:
> 
> Demorep wrote:
> >After rethinking the problem of Condorcet circular ties, I note
> >that such ties occur because any tiebreaker method using additional
> >rankings beyond the first choice ranking has the potential for
> >strategic voting
> -snip-
> 
> The Gibbard-Satterthwaite theorem points out that *any* method has
> the potential for strategic voting.  Demorep's proposed new method
> is even worse than Condorcet and Smith//Condorcet.
> 
> I suggest that when people think about a new method they should
> first test them on the following two examples:
> 
>      46:ABC                          46:A
>      10:BAC                          10:BAC
>      10:BCA                          10:BCA
>      34:CBA                          34:CBA
>      ------                          ------
>      MUST ELECT B                    MUST NOT ELECT A
> 
> In the example on the left, it should elect B (the compromise
> centrist who would beat any other in a head to head matchup).  In
> the example on the right, it should not elect A, else the supporters
> of A have a clear incentive to strategize.  If truncation would
> elect A, the method has a serious problem.
> 
> We may as well call this two-example test a criterion: the
> "Truncation Resistance #2" criterion, perhaps.  Do we have a
> consensus that any method which fails TR-2 should not be posted
> here in EM, unless it's accompanied by a solid explanation why it's
> better than Condorcet or Smith-Condorcet on some other criterion we
> should respect?  Such a consensus would save us all a lot of time.
> 
> If I understand it right, Demorep's new "First Choices tiebreaker"
> method fails TR-2.  Though it passes the left example (elects B),
> it fails the example on the right: the circular tie is resolved by
> eliminating B (which has fewest first choices), then A beats C.
> 
> ---Steve     (Steve Eppley    seppley at alumni.caltech.edu)