The variance of a continuous random variable X is defined as

where μ = E[X]. The variance is a measure of the dispersion of the probability distribution and is usually denoted by the symbol σ². An alternative expression which makes calculation of the variance easier is given by

That is, the variance is the mean of the squares less the square of the mean. Another useful measure of dispersion is the standard deviation, which is defined as the positive square root of the variance. (Denoted by σ). Note the similarity between the above and the corresponding discrete definition.