[EM] DH3 pathology, margins, and winning votes

[Dave Ketchum]

Warren talks of a "DH3 pathology" as a serious problem that should 
encourage Condorcet backers, such as myself, to abandon ship and switch to 
IRV:
      The pathology can occur with certain patterns of votes AND plotting.
      IRV has problems that can occur without plotting.
      So, NO SALE!

Restating pathology:
      Expectable vote counts are known magically.
      There is a near tie among 3 or more candidates.
           Takes this to make plotting practical.
           Takes this and a cycle pattern to have cycle complications.
      Likely cycles get involved - they sort out many near ties, but exact 
required votes are hard to predict and cause.
      Backers of one of the losers can agree to switch votes to a 
different loser to cause a more desirable loser to become winner (to 
switch to X to cause X to win would not be pathology).
      If others find out and do counter-plotting, perhaps one neither 
likes becomes winner - no tears from here for this negative accomplishment.

IRV problem:
      30 A
      25 C>B
      23 D>B
      22 B

Here Condorcet sees B as clear winner, with exact counts not mattering - C 
and D counts will not matter until one competes with B; take a big change 
for A to compete.

IRV sees A as winner with 30%.  ONE voter voting B instead of D>B would 
make B win.

DWK

On Sat, 26 Aug 2006 12:57:16 -0400 Warren Smith wrote:
> Sorry, for some reason, the hyperlink in my previous post was omitted.
> Let me try again:
>    http://rangevoting.org/WinningVotes.html
> 
> This is a web page that discusses "winning votes" versus "margins" and
> includes a careful look at the "DH3 pathology" which is a very serious
> problem faced by both Borda and every Condorcet method.  It is shown that
> DH3 still clobbers Condorcet methods even if they allow equal rankings and
> even if they use winnign-votes or margins.
> 
> The reason DH3 is "very serious" is it has the maximally bad effect (elects worst canddt)
> and it has near maximal commonness (merely need 3 rivals and a dark horse running).
> 
> Perhaps you all will have some comments.  I'm quite sure I have not included all
> the wisdom about wv versus margin that EM has accumulated.   But on the
> other hand I also feel those at EM have underestimated or trivialized the power of
> the DH3 pathology to cause massive destruction.
> Warren D Smith
> http://RangeVoting.org
-- 
  davek at clarityconnect.com    people.clarityconnect.com/webpages3/davek
  Dave Ketchum   108 Halstead Ave, Owego, NY  13827-1708   607-687-5026
            Do to no one what you would not want done to you.
                  If you want peace, work for justice.