[EM] Re: Condorcet elimination PR

[Chris Benham]

James G-A,
Further  on the subject of my proposal  to modify STV-PR by , after 
transfering supluses, eliminating the Condorcet loser among those votes 
and fractions of votes not tied up in quotas: I am afraid I have 
discovered that this method fails FBC (Favourite Betrayal Criterion).
To demonstrate the problem, I have slightly modified your example by 
introducing  candidate X.

300 votes,  3 seats,  Droop quota = 75.

74   A B C D X E
39   B A C D X E
75   C
37   D X E C B A
73   E D X C B A
  2   X E D C B A

Compared to your example, all that has changed is that X has  stood and 
taken  2 first preference votes from D and has occupied  the runner-up 
Condorcet loser position. The result, CBD, is unchanged.
The problem is that if  those 2  X  supporters had reversed their top 
two preferences, then E would have got a quota and the result would have 
been CEB. By insincerely down-ranking their favourite, those two voters 
could have caused their second prefernce to be elected instead of their 
third.
I'd be surprised if  this sort of problem is completely avoidable with 
ranked-ballot PR. Do you know how "Sequential STV" and "CPO-STV" fare in 
this respect?

Chris Benham