[EM] I need an example of Condorcet method being subjected to the spoiler effect if any

[robert bristow-johnson]

On Jan 21, 2010, at 2:29 AM, Kristofer Munsterhjelm wrote:

> robert bristow-johnson wrote:
>> i think that the answer is "no", if a Condorcet winner exists and  
>> that all bets are off if a CW does not exist, except, perhaps for  
>> these "strategy-resistant" methods such as Markus Schulze's  
>> method.  i sorta understand it, but since he hangs here, i think  
>> Markus should address the question if the Schulze method is  
>> spoiler free.
>
> MAM/Ranked Pairs is also pretty strategy-resistant and is easier to  
> understand. Schulze has the advantage of producing better results  
> in some cases (closer to Minmax), but if "ability to describe to  
> the public" is important, then Ranked Pairs wins there.
>
>>> Could someone please provide me with an example of the spoiler  
>>> effect occuring with the Condorcet method  of counting rank  
>>> choice ballots or tell me why the spoiler effect doesn't happen  
>>> with Condorcet in  a few words?
>
> (...)
>
>> i would say that (with the CW existing), it's spoiler-proof.
>
> Yes. If there's a CW and a candidate is added, and that candidate  
> doesn't create a cycle, then the winner doesn't change.

and if that candidate that is added doesn't *win*.  a spoiler is  
*not* a winner that when removed from the election and all ballots  
does not change who the winner is.  a spoiler must be a loser to the  
election, whose presence changes who the winner is.  i remember  
reading someone's poor attack on IRV (in local Burlington blogs) that  
claimed that Bob Kiss (who won Burlington's IRV election) was a  
"spoiler".  they were misusing or misunderstanding the concept of a  
3rd-party candidate.  (maybe somewhere else, a 3rd-party candidate  
can only hope to be a spoiler, but in Burlington a 3rd-party  
candidate can expect to win office once in a while.  that's a little  
different.)

so, we have a CW...  add a candidate, if that candidate does not  
become the winner, nor cause a cycle, then the Condorcet Winner we  
had before continues to be the CW with the added candidate.  (boy, i  
guess we're rephrasing the same thing multiple times!)

> All the "tricky" stuff happens when there is a cycle, or the  
> candidate makes one.

yup.

> The advanced methods can claim further resistance: Schulze and  
> Ranked Pairs both make their winner decision independent of  
> candidates not in the Smith set.

this, i understand...

> River is independent of Pareto-dominated alternatives - a candidate  
> is Pareto-dominated if everybody who ranks both him and some other  
> (specific) candidate, rank the other candidate above him (e.g. X is  
> Pareto-dominated by Y if all voters who rank both X and Y say Y>X,  
> and there's at least one such voter).

i wouldn't mind if someone explains this.  i don't know what "Pareto- 
dominated" is about.  can someone expound?

> I imagine these resistances would mostly come into play in smaller  
> elections. Still, they're nice to have, and their existence  
> immediately tells parties not to try exploiting certain weaknesses  
> (because it won't work).

yes, that's the whole point.  this is why i am not yet afraid of  
someone strategically voting to push a Condorcet election into a  
cycle.  it would be an unsafe way to accomplish a political goal.   
how could anyone predict what would happen?

--

r b-j                  rbj at audioimagination.com

"Imagination is more important than knowledge."