In intuitive terms, variance is a measure of the spreading of the data set from its mean. The variance of a data set can be found with:

The Standard Deviation is the square root of the mean.

The variance of is defined by

and provides a measure of how much varies around its mean value

Expanding out the square, we see that the variance can also be written in terms of the expectations of and :

In particular, we can consider the variance of the variable itself, which is given by: