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Contents
require 'facets/math/sum'
require 'facets/math/mean'
require 'facets/math/approx_equal'
module Math
# Calculates the Theil index (a statistic used to measure
# economic inequality).
#
# TI = \sum_{i=1}^N \frac{x_i}{\sum_{j=1}^N x_j} ln \frac{x_i}{\bar{x}}
#
# http://en.wikipedia.org/wiki/Theil_index
#
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
end
Version data entries
9 entries across 8 versions & 2 rubygems