[EM] Re: Later-no-harm question

[Chris Benham]

  On Mon. Dec.22, 2003 ,  Bart  Ingles wrote:

"I can't see much value in LNH as a criterion, unless getting voters to
rank as many candidates as possible is seen as an end in itself."

Meeting Later-no-harm is logically the same thing as being truncation-proof.

All methods that meet Later-no-harm must logically also meet Mono-add-top, but 
not vice versa.
"Mono-add-top" is a Woodall criterion which says that adding ballots that all give
first-preference to X must not harm X. It is met by IRV and Margins, but not by WV.

In trying to change from Plurality to a ranked-ballot method, Later-no-harm is an 
excellent selling-point to plurality-minded voters.

I think Later-no-harm should be considered together with Later-no-help. The two should
be in balance, ie the chances of a lower-preference helping or harming a higher-preference
should be the same. Otherwise "zero information strategy" (as something different from
sincere voting) can raise its ugly head. A method in which the chance of helping a higher
preference (by ranking a candidate) is greater than the chance of harming, can create
incentive to "random fill". A method such as Woodall's "Descending Acquiescing Coalitions"
(DAC) which fails Later-no-harm but meets Later-no-help might be Approvalish. His method
"Descending Solid Coalitions"(DSC) meets Later-no-harm but fails Later-no-help.
Another of his methods,"Descending Half-solid Coalitions"(DHSC), which actually fails both
criteria, might be preferable.

Chris Benham  


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