Measures of dispersion describe how similar or different scores in a data set are from each other. The more similar the scores, the lower the measure of dispersion, and the more varied the scores, the higher the measure. Common measures include the range, semi-interquartile range (SIR), variance, and standard deviation. Variance is the average of the squared deviations from the mean, while standard deviation is the square root of variance. Skew and kurtosis also describe the shape and symmetry of a distribution.