In statistics, the variance (or population variance) is the mean average of the squared distance between each data point and the arithmetic mean. We get it by squaring the standard deviation:

In probability theory, we similarly have a formula:

In statistical learning, the variance is the amount by which would change if it was estimated using a different training data set.

Intuitively, the variance describes how far members are from the mean (overall spread). The standard deviation describes the average distance.

Properties

For a constant :