Informally it measures how far a set of numbers is spread out from their average value. Subtract the Mean and square the result the squared difference. The formula for variance is.
Variance is the average of the squared differences from the mean.
For a discrete random variable X the variance of X is written as Var X. It is calculated by taking the average of squared deviations from the mean. To calculate the variance follow these steps. In probability theory and statistics covariance is a measure of the joint variability of two random variables.