statBeginner statBeginner - 1 year ago 136
Python Question

Multivariate Normal CDF in Python using scipy

In order to calculate the CDF of a multivariate normal, I followed this example (for the univariate case) but cannot interpret the output produced by scipy:

from scipy.stats import norm
import numpy as np
mean = np.array([1,5])
covariance = np.matrix([[1, 0.3 ],[0.3, 1]])
distribution = norm(loc=mean,scale = covariance)
print distribution.cdf(np.array([2,4]))

The output produced is:

[[ 8.41344746e-01 4.29060333e-04]
[ 9.99570940e-01 1.58655254e-01]]

If the joint CDF is defined as:

P (X1 ≤ x1, . . . ,Xn ≤ xn)

then the expected output should be a real number between 0 and 1.

Answer Source

After searching a lot, I think this blog entry by Noah H. Silbert describes the only readymade code from a standard library that can be used for computing the cdf for a multivariate normal in Python. Scipy has a way to do it but as mentioned in the blog, it is difficult to find. The approach is based on a paper by Alan Genz’s.

From the blog, this is how it works.

from scipy.stats import mvn
import numpy as np
low = np.array([-10, -10])
upp = np.array([.1, -.2])
mu = np.array([-.3, .17])
S = np.array([[1.2,.35],[.35,2.1]])
p,i = mvn.mvnun(low,upp,mu,S)
print p