[EM] criteria compliance and strategic vulnerability

Chris Benham chrisbenham at bigpond.com
Mon May 23 11:57:32 PDT 2005

Kevin (and James G-A),
You wrote (Sat.May21):

>I don't recall off-hand which "consistency" criteria James has.
They are Monotonicity, Participation and  Consistency.

>My opinion
>of MAPlump and MAppend is that they should be awfully easy to satisfy in most
>cases. Monotonicity implies MAppend. MAPlump just says "adding in bullet votes
>for the winner can't make him lose."
>Looking at "Properties of single-winner election rules," not a single method
>that anyone has here proposed fails either of those criteria. 
That's not quite true. It  isn't the case that any plausible-looking 
method easily meets those criteria automatically.
Last year (Thu.Dec.2) James G-A  suggested  a version of completing 
Condorcet by IRV  that failed  Mono-ad-Plump and Mon-append. It involved 
eliminating (dropping from the
ballots)  any candidates not in the Smith (or Schwartz?) set and also 
any candidates that have a full majority pairwise defeat unless they all 
do; and then electing the winner of the IRV
count among the remaining candidates.

Here is proof  from Douglas Woodall. ("AV" is the Alternative Vote, the 
UK name for IRV).

 >abcd 10
 >bcda  6
 >c     2
 >dcab  5
 >All the candidates are in the top tier, and the AV winner is a.  But
 >if you add two extra ballots that plump for a, or append a to the two
 >c ballots, then the CNTT becomes {a,b,c}, and if you delete d from all
 >the ballots before applying AV then c wins.

Chris  Benham

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