[EM] Dual Dropping method and "Preference Approval" ballot ideas

[Markus Schulze]

Dear Matt,

in so far as the way DD chooses between the SSD winner
and the RP winner isn't clone-proof DD isn't clone-proof.

******

It isn't clear whether DD meets monotonicity. Consider the
following scenario:

  Candidate A is the SSD winner. Candidate B is the RP winner.
  DD chooses candidate A.

  Suppose, some voters improve their opinion about candidate A.
  Then, as SSD meets monotonicity, candidate A is still the SSD
  winner. However, it is possible that, by ranking candidate A
  higher, the RP winner is changed from candidate B to candidate
  C. When now DD chooses candidate C, then DD violates monotonicity.

****** 

Therefore, I suggest that pure RP or pure SSD should be used
instead of DD.

By the way, the reason why I have proposed SSD (aka beatpath
method, aka path voting, aka Schulze method) as an alternative
to RP is that the SSD winner differs from the MinMax winner
significantly less frequently than the RP winner differs from
the MinMax winner. The MinMax method is the best Condorcet
method when strategical nomination is not an issue.

Markus Schulze

----
For more information about this list (subscribe, unsubscribe, FAQ, etc), 
please see http://www.eskimo.com/~robla/em