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bin/gems/facets-3.1.0/lib/standard/facets/math/atkinson_index.rb

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+require 'facets/math/theil_index'
+
+module Math
+
+  # Closely related to the Theil index and easily expressible
+  # in terms of it.
+  #
+  # AI = 1-e^{theil_index}
+  #
+  # http://en.wikipedia.org/wiki/Atkinson_index
+  def self.atkinson_index(array)
+    t = theil_index(array)
+    (t < 0) ? -1 : 1-Math::E**(-t)
+  end
+
+end