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: