[EM] STAR

[Toby Pereira]

 Chris
It's not that I disagree with your views of STAR's behaviour as a method. And there are changes that could be made that would improve STAR, as you say. However, I just don't think that STAR's failure here can reasonably be called a monotonicity failure.
Toby
    On Saturday, 19 August 2023 at 04:10:32 BST, C.Benham <cbenham at adam.com.au> wrote:  
 
  
Toby,
 
 

I wouldn't count this as a monotonicity failure because it involves decreasing Y's score as well as increasing X's.
 
  
 This is like a sophist's technical loophole.  Why do we particularly care about "monotonicity failure"? To avoid some hypothetical mild embarrassment?  
 For the sake of marketing bragging rights? 
 
 Or because it is related to Push-over strategy incentive/vulnerability?  STAR is  much worse in that respect than IRV because there the strategists are entirely
 relying on other voters to both get their favourite into the final two and to there win the pairwise contest, so if too many of X's supporters try the strategy it
 could backfire. 
 
 Whereas with STAR the strategists could be a bit cautious and give the weak candidate they are trying to promote into the final a score of max. minus one
 while also giving their favourite X max. points.
 
 That way all of X's supporters could use the strategy and it could still succeed. 
 
 The 0-5 score ballot is too restrictive (certainly for STAR)  Say, as I earlier advocated, the voters rank however many candidates they want to and give an approval cutoff wherever
 they want, and we elect the pairwise winner between the two most approved candidates.
 
 That would be very similar to STAR (0-5 score ballots) but wouldn't it be better?  And also a method that fails mono-raise and Condorcet and many other criteria 
 and is obviously terrible?
 
 Chris
 On 17/08/2023 9:47 pm, Toby Pereira wrote:
  
 
 I wouldn't count this as a monotonicity failure because it involves decreasing Y's score as well as increasing X's. Mono-raise may have been defined specifically for ordinal ballots where raising a candidate inevitably pushes others down. Whereas with a rated ballot, I think one would be more likely to define monotonicity criteria in terms of increasing a candidate's score while leaving all others the same. 
  Toby 
  
      On Thursday, 17 August 2023 at 05:43:00 BST, C.Benham <cbenham at adam.com.au> wrote:  
  
     
Toby Pereira wrote: 
 
I'm not a fan of STAR, but I am still interested in seeing how it stands up to scrutiny given that it has a following. (Actually I'm not aware of how STAR fails monotonicity. I was under the impression that it passed.) 
  
 
Toby,
 
 To give you a bit of a preview before I get around to cooking up all the examples, nothing with such obvious Push-over incentive can meet mono-raise (aka "monotonicty")
 
 Suppose  X beats Y in the final.   Now suppose on some ballots with Y above X, we raise X so it is now above Y.  That could reduce Y's score enough for it to be replaced in the final
 by Z, a candidate that pairwise beats X.
 
 Voters who are mainly concerned to have their favourite X win and are fairly certain that X will reach the final will have a strong incentive to give X max points (5) and then also
 give a 4 (or even a 5) to all those candidates that they think X can beat pairwise.
 
 
If enough voters use that strategy and it fails, both the finalists could be candidates with little sincere support.
 
 Chris Benham
 
 
  O         
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