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.
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