[EM] IRV vs Condorcet

[robert bristow-johnson]

On May 26, 2010, at 8:19 PM, Dave Ketchum wrote:

> At least some should see what I write here about IRV vs Condorcet.   
> Both use ranked ballots, with almost identical rules for voters  
> (Condorcet permits equal ranking).

assuming that few voters would make equal rankings if allowed to (a  
big assumption), this is why we were able to see how the Burlington  
2009 election would have turned out if it were Condorcet instead of IRV.

> IRV voters can express their desires, but vote counters are not  
> required (in fact not even permitted) to count all that they have  
> voted.  Steps:
> 1. Stack ballots per top rank vote.  If one stack has a majority of  
> the stacked ballots, this is winner.
> 2. For each ballot in smallest stack, remove one or more names to  
> get to a stack to move it to.
> 3. Ballots that have no stack to move to are done.
> 4. Back to step 1 with remaining ballots.
>
> Problems:
>     Only the top rank candidate on each ballot is seen at any time -  
> lower ranks do not get considered until becoming top rank.
>     Once a candidate has been smallest stack, or never had a stack,  
> any votes seen for that candidate get ignored.
>     Backers make a big deal of "majority" - but it is of the final  
> stacks, not of all ballots.

what it is, is *a* majority.  for a particular pair that is left  
standing after the other candidates are eliminated by the IRV STV  
rules (which is the essential problem with IRV).  assuming no ties,  
each pair of candidates drawn from the candidate pool has an intrinsic  
majority.  the question is: which majority is the salient majority?

>     Suppose Tom, Dick, and Harry share all the top rank votes, and  
> Joe gets all the 2nd rank.  Then if raced in pairs Joe would get  
> twice the votes of each of them - but Joe is invisible in IRV.

or, we could change Joe's name to "Andy" and Tom and Dick to "Bob" and  
"Kurt", leave Harry out of it, and this hypothetical becomes less  
hypothetical.

--

r b-j                  rbj at audioimagination.com

"Imagination is more important than knowledge."