In probability theory and statistics, the variance is a way to measure how far a set of numbers is spread out.Variance describes how much a random variable differs from its expected value. The variance is defined as the average of the squares of the differences between the individual (observed) and the expected value. This means that it is always positive. A variance is often represented by the symbol \displaystyle{ \sigma^2 }, if the data is the entire population, and \displaystyle{ s^2 }, if the data is from a sample.