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# Graphical representation of the portfolio diversification effect

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:

Year

Stock A

Stock B

2009

0.102

0.1062

2010

-0.02

0.23

2011

0.213

0.045

2012

0.12

0.234

2013

0.13

0.113

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) ...```

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