[EM] Trying to define "Later-no-harm for viable candidates criterion" (Re: full rankings, voter desire for)

[Kevin Venzke]

Rob,

I'm responding just quickly:

--- Rob Lanphier <robla at robla.net> a écrit :
> On Sun, 2005-10-16 at 22:47 +0200, Kevin Venzke wrote:
> > I don't know of a way to weaken LNHarm which would still result in a guarantee
> > that voters could "take to the bank."
> 
> My hope would be that we can come up with a system where voters could
> feel comfortable ranking all but one of the viable candidates.  So, if
> we end up in a situation like we were at one point in 1992, where
> Clinton, Bush and Perot were all viable candidates, voters could feel
> comfortable ranking two out of three of them, without worrying at all
> about helping anyone defeat their first choice.  For such a system, we
> could then recommend that voters do not rank anyone below their least
> favorite viable candidate (which would be a very minimal amount of
> strategy to impose).
> 
> So, the partial definition of Later-no-harm for viable candidates
> criterion" (LNHarmVC) could be:
> "Adding a /viable/ preference to a ballot must not decrease the
> probability of election of any candidate ranked above the new
> preference."
> 
> The trick, of course, is to define "viable" in mathematical terms in
> such a way that matches the popular view of viability.
> 
> A simple, but probably incorrect, definition would be "any candidate who
> is ranked on a majority of ballots".  I would hope we could come up with
> a less stringent definition, because that would potentially mean that a
> candidate in a close, polarized three way race might not be "viable" by
> the definition.  An alternative definition might be "any candidate who
> could win without violating Plurality".

The problem I see with the latter is that it doesn't seem to get us much.
The main phenomenon with LNHarm failures is that you list an additional
candidate, and this causes this candidate to win instead of someone you
liked better. Usually this isn't a *weak* candidate. Weak candidates can win
under MMPO mainly because MMPO doesn't measure how *good* such candidates do,
only how bad they're hit by other candidates. If MMPO measured weak
candidates' performance against other candidates, we'd clearly have to fail
LNHarm, because by listing this weak candidate as a lower preference, we
would inherently be elevating him (i.e. not just above the unranked candidates).

As far as the majority requirement... This seems to create a large class of
situations in which the voters debate whether listing the new preference
will cause that candidate to have a majority, in which case LNHarm isn't
guaranteed to them.

Actually, the Plurality requirement has the same issue:

48 A
25 B (>C)
27 C>B

C is barred by Plurality, but the B voters can change this. I assume B must
win when B voters don't give C the second preference. (Electing A or C seems
very undesirable for most purposes.) In that case, it's not possible to give
the B voters a LNHarm assurance, since the C voters would have the same claim
to it.

What do you think of this scenario?

Kevin Venzke



	

	
		
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