Standard deviation

standard_deviation.rb

module Enumerable

    def sum
      self.inject(:+)
    end

    def mean
      self.sum/self.length.to_f
    end

    def sample_variance
      m = self.mean
      sum = self.inject(0){|accum, i| accum +(i-m)**2 }
      sum/(self.length - 1).to_f
    end

    def std_dev
      return Math.sqrt(self.sample_variance)
    end

end