statBeginner - 7 months ago 93

Python Question

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

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
0.2881578675080012
```