code: def normcdf(x, mu, sigma):
      
      t = x-mu
      y = 0.5*erfcc(-t/(sigma*math.sqrt(2.0)))
      if y > 1.0:
          y = 1.0
      return y
docs: Describes the probability that a real-valued random variable X with a given
      probability distribution will be found at a value less than or equal to X
      in a normal distribution.
      
      http://en.wikipedia.org/wiki/Cumulative_distribution_function