Solved – Why does independence imply zero correlation

correlationcovarianceindependencemathematical-statistics

First of all, I'm not asking this:

Why does zero correlation not imply independence?

This is addressed (rather nicely) here: https://math.stackexchange.com/questions/444408/why-does-zero-correlation-not-imply-independence

What I'm asking is the opposite…say two variables are entirely independent of one another.

Couldn't they have a tiny bit of correlation by accident?

Shouldn't it be…independence implies VERY LITTLE correlation?

Best Answer

By the definition of the correlation coefficient, if two variables are independent their correlation is zero. So, it couldn't happen to have any correlation by accident!

$$\rho_{X,Y}=\frac{\operatorname{E}[XY]-\operatorname{E}[X]\operatorname{E}[Y]}{\sqrt{\operatorname{E}[X^2]-[\operatorname{E}[X]]^2}~\sqrt{\operatorname{E}[Y^2]- [\operatorname{E}[Y]]^2}}$$

If $X$ and $Y$ are independent, means $\operatorname{E}[XY]= \operatorname{E}[X]\operatorname{E}[Y]$. Hence, the numerator of $\rho_{X,Y}$ is zero in this case.

So, if you don't change the meaning of correlation, as mentioned here, it is not possible. Unless, clarify your defintion from what the correlation is.