A measure of the variability in the values of a random variable. In context the word ‘population’ is often omitted. It is defined as the expected value of the squared difference between the random variable and its expected value:

where E(X) denotes the expected value of the random variable X. For a discrete random variable X, taking values x₁, x₂,…, x_n, the variance of X can be calculated as follows:whereFor a continuous random variable X, with probability density function f, the variance of X can be calculated as follows:whereThe term ‘variance’ was coined by Sir Ronald Fisher in 1918. Fisher used the symbol σ², since the variance was the square of the standard deviation that Karl Pearson had denoted by σ in 1894. See Covariance.