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

[Ross Hyman]

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.