## Formula:

$$Cov (X,Y) =\frac{\sum (X_i - \overline X)(Y_i - \overline Y)}{n-1}$$

Where :

• xi = Data variable of x
• yi = Data variable of y
• x = Mean of x
• y = Mean of y
• n = Number of data variables

Example :

Suppose that the closing prices of two stocks daily basis is given below:

day abc return xyz return
1 2.8 1.5
2 2.5 3.3
3 1.1 3.6
4 1.4 3.1
5 0.4 2.2

Calculate the mean(average):

$$\text{Mean(abc)} = \frac{2.8 + 2.5 + 1.1 + 1.4 + 0.4}{5}$$

$$\text{Mean(abc)} = 1.64$$

$$\text{Mean(xyz)} = \frac{1.5 + 3.3 + 3.6 + 3.1 + 2.2}{5}$$

$$\text{Mean(xyz)} = 2.74$$

$$Cov (X,Y) =\frac{\sum (X_i - \overline X)(Y_i - \overline Y)}{n-1}$$

Put the values into the formula

Cov(x,y) = (((2.8 – 1.64) * (1.5 – 2.74)) + ((2.5 – 1.64)*
(3.3 – 2.74)) + ((1.1 – 1.64) * (3.6 – 2.74)) +
(1.4 – 1.64) * (3.1 –2.74) + ((0.4 – 1.64) *
(2.2 – 2.74))) / (5 – 1)

$$Cov(x,y) = \frac{-1.44 + 0.48 - 0.46 - 0.08 + 0.67}{4}$$

$$Cov(x,y) = \frac{- 0.83}{4}$$

$$Cov(x,y) = -0.20$$