code: def normpdf(x, mu, sigma):
      
      u = (x-mu)/abs(sigma)
      y = (1/(math.sqrt(2*pi)*abs(sigma)))*math.exp(-u*u/2)
      return y
docs: Describes the relative likelihood that a real-valued random variable X will
      take on a given value.
      
      http://en.wikipedia.org/wiki/Probability_density_function