Deviation is a measure of difference between the observed value of a variable and some other value, often that variable's mean. The sign of the deviation positive or negative (+ or -), reports the direction of that difference. The deviation is positive when the observed value exceeds the reference value. The magnitude of the value indicates the size of the difference.

**Statistics of the distribution of deviations are used as measures of statistical dispersion.**

Standard deviation is the frequently used measure of dispersion: it uses squared deviations, and has desirable properties, but is not robust. Average absolute deviation, is the sum of absolute values of the deviations divided by the number of observations.** **Median absolute deviation is a robust statistic which uses the median, not the mean, of absolute deviations.** **Maximum absolute deviation is a highly non-robust measure, which uses the maximum absolute deviation.