In finance, we could remove firm-specific risk by combining different stocks in our portfolio. First, let us look at a hypothetical case by assuming that we have 5 years' annual returns of two stocks as follows:

We form an equal-weighted portfolio using those two stocks. Using the mean() and std() functions contained in NumPy, we can estimate their means, standard deviations, and correlation coefficients as follows:

>>>import numpy as np
>>>A=[0.102,-0.02, 0.213,0.12,0.13]
>>>B=[0.1062,0.23, 0.045,0.234,0.113]
>>>port_EW=(np.array(ret_A)+np.array(ret_B))/2.
>>>round(np.mean(A),3),round(np.mean(B),3),round(np.mean(port_EW),3)
(0.109, 0.146, 0.127)
>>>round(np.std(A),3),round(np.std(B),3),round(np.std(port_EW),3)
(0.075, 0.074, 0.027)

In the preceding code...