Defined as the average of the absolute differences of each value in a set of values to the average of all the values in that set. Absolute differences is the difference between two values expressed without plus or minus sign. For example, the average or mean of the set of values 1, 2, 3, 4, and 5 is (15/5) or 3. The difference between this average value (3) and each value in the set is 2, 1, 0, -1, and -2. The absolute differences are 2, 1, 0, 1, and 2. The average of these numbers (6 / 5) is 1.2 which is the mean deviation. Also known as mean absolute deviation. It is a measure of dispersion when the number of values or quantities is small, otherwise use standard deviation.