# File lib/ai4r/data/statistics.rb, line 29
      def self.variance(data_set, attribute, mean = nil)
        index = data_set.get_index(attribute)
        mean = mean(data_set, attribute)
        sum = 0.0
        data_set.data_items.each { |item| sum += (item[index]-mean)**2 }
        return sum / (data_set.data_items.length-1)
      end