[EM] question/comments re DMC

[Chris Benham]

Warren,
You and Jobst  recently had this exchange about  Definite Majority 
Choice (DMC) and  Range voting. You begin by quoting
one of  Jobst's  "15  reasons to support DMC".

>6. Robustness against "noise" candidates.. cloneproof...
>  
>
>WS: > --also true of range.
>  
>
>
>JH: Could you say more precisely what you mean here?
>
>
>WS:--Range voting is immune to clones in the sense that any number of cloned
>candidates, all of whom get the same rangevote scores, can be added to the scene,
>and the election winner will remain unchanged (except perhaps for replacement by a clone).
>
>Also, if "noise" candidates are added who have no hope of winning, then the range votes
>with noise scores being adjoined  for the new candidates, will still yield the same winner
>and indeed the same totals.
>
>Many other voting schemes have these properties (and many also do not
>have these properties) but in range's case it is particularly self-evident.
>
Range  only has  this property in a technical sense, in a way that is 
connected with its technical failure of  May's axiom, i.e. it
doesn't reduce to FPP when there are only two candidates.

Suppose that in the period leading up to the election it is known for 
sure that  two candidates will stand, A and B.  A is a left-wing
candidate that is hated by big money and its mass media. B is a 
centre-right candidate that they like. Reliable but perhaps not widely
published polls give  A52%,  B48%.  The method to be used in the 
election is Range, and with just these two candidates standing
the voters have no reason not to give maximum points to their preferred 
candidate and minimum to the other resulting in a solid win
for  A. 
How to change B from being the majority loser to the "super-majority  
consensus candidate"?  Easy! A third candidate, C is nominated.
C is  a  horror far-right candidate. Maybe some of  A's supporters are 
members of some ethnic/racial/religious minority  that the C
candidate says he's in favour of  persecuting. Anyway, now all the mass 
media have to do now is to convince some of  A's supporters
that C has some chance of winning the election, or just that they should 
give a maximised sincere vote.

So without  C we have:
52: A99, B0
48: B99, A0
A wins 5148  to B4752.

With C added  and some of  A's supporters conned and/or frightened, this 
could become:
47: A99, B0, C0
05: A99, B98, C0
46: B99, A0, C0
02: C99, B98, C0

Now B wins:  B5242,  A5148,  C198.   (Approval is also vulnerable to 
this scenario.)
Note that in this example the voted and sincere (binary) pairwise 
preferences are  A>B 52-48,  A>C 52-2, B>C 98-2.

I  think DMC is a very very good (possibly the best) single-winner 
method to propose for public office elections if we insist on  Condorcet
and  Mono-raise.


Chris  Benham

.



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