Mean Deviation may be defined as the arithmetic average of the deviations of the items of a series taken from its central value ignoring the plus and minus signs.Here, the central value may be type of average value but preference is given to Mean, Median and Mode. Again, among these three averages, Median is the most preferable one because of the following two reasons:
(i) Total of deviations taken from median ignoring plus and minus signs is the minimum possible.
(ii) Median, invariably, gives a round figure (except in case of continuous series of course) for which the figures of deviations taken from the Median become round, and simple to work out.
However, in practice, arithmetic average is more frequently used in calculating mean deviations because, averages in common parlance means arithmetic average.
The most important essential feature of mean deviation is that while taking the deviations of the various items from their central value, the plus and minus signs are ignored. This is a serious charge that is leveled against the mean deviation for that it amounts to violation of the algebraic principle. But this is done is order to avoid mutual cancelling effect of the + and – signs which reduces the sum of the deviations may become zero in certain cases, and thereby it may not be possible to find out the average of such deviations.
As already pointed out, Mean Deviation can be calculated in both absolute and relative manners. The fundamental formula for its computation stands as under:
M.D. (8) = ∑D/N
Where, D (read as Mod.D) = Modulus value or absolute value of Deviation from Mean or Median or Mode ignoring ± signs.
And M.D. = Mean Deviation.