Which one of the following expressions is not correct to calculate the coefficient of correlation between two variables?
1
\(\rm\frac{\sum x y}{N \cdot \sigma x \cdot \sigma y}\)
2
\(\rm\sqrt{(b y x)(b x y)}\)
3
\(\rm\frac{Cov(x, y)}{\sigma_x^2 \cdot \sigma_y^2}\)
4
\(\rm\frac{Cov(x, y)}{\sigma_x \cdot \sigma_y}\)