code: def stdev(x):
      r
      from math import sqrt
      n = len(x)
      mean = sum(x) / float(n)
      std = sqrt(sum((a - mean)**2 for a in x) / float(n - 1))
      return std
docs: r"""Calculate standard deviation of data x[]:
          std = sqrt(\sum_i (x_i - mean)^2 \over n-1)
          https://wiki.python.org/moin/NumericAndScientificRecipes