7.1.5 Mean Deviation
If xi/fi, I = 1,2, …..n is the frequency distribution, then mean deviation from the average A (usually mean, median or mode) is given by
Mean deviation =
Where represents the modulus or the absolute value of the deviation (xi – A), when the –ve sign is ignored.
Since mean deviation is based on all the observations, it is a better measure of dispersion than range or quartile deviation. But the step of ignoring the signs of the deviation (xi-A) creates artificiality and renders it useless for further mathematical treatment.
It may be pointed out here that mean deviation is least when taken from median.