# 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