APA Dictionary of Psychology

a measure of variability in a data set calculated as the average of the distances between each score and each of the other scores, disregarding whether the deviation is positive or negative. For example, consider the following three scores: 1, 3, and 9. The mean difference would be calculated as (|1−3| + |1−9| + |3−1| + |3−9| + |9−1| + |9−3|)/6 = (2 + 8 + 2 + 6 + 8 + 6)/6 = 32/6 = 5.33.