def skew m=nil
          if @data.respond_to? :skew
            @data.skew
          else
            m ||= mean
            th  = @data.inject(0) { |memo, val| memo + ((val - m)**3) }
            th.quo((size - indexes(*Daru::MISSING_VALUES).size) * (standard_deviation_sample(m)**3))
          end
        end
Calculate skewness using (sigma(xi - mean)^3)/((N)*std_dev_sample^3)