The average deviation is one of several indices of variability that statisticians use to characterize the dispersion among the measures in a given population.

To calculate the average deviation of a set of scores it is first necessary to compute their mean and then specify the distance between each score and that mean without regard to whether the score is above or below the mean. The average deviation is defined as the mean of these absolute values.

Algebriacally the average deviation is specified as follows: