[EM] an entropy formula for the effective number of parties

Jameson Quinn jameson.quinn at gmail.com
Fri Dec 14 05:28:37 PST 2012

Interesting. When is it different from the other formula?


2012/12/13 Ross Hyman <rahyman at sbcglobal.net>

> Here is a physics alternative to the "effective number of parties"
> formulas mentioned on the Wikipedia page:
> http://en.wikipedia.org/wiki/Effective_number_of_parties
> Based on the concept of entropy, a sensible formula for the effective
> number of parties = exp(-sum_i P_i log(P_i))
> where P_i is the portion of the votes or portion of seats for party i.
>  sum_i P_i =1.
> It is sensible because for an election where n parties get 1/n of the vote
> each and the rest of the parties get zero votes, the effective number of
> parties from the entropy formula is n.
> ----
> Election-Methods mailing list - see http://electorama.com/em for list info
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