# File lib/more/facets/math/theil_index.rb, line 14 def self.theil_index(array) return -1 if array.size <= 0 or any? { |x| x < 0 } return 0 if array.size < 2 or all? { |x| approx_equal(x, 0) } m = mean(array) s = sum(array).to_f inject(0) do |theil, xi| theil + ((xi > 0) ? (log(xi.to_f/m) * xi.to_f/s) : 0.0) end end