Mean Deviation
The two methods of dispersion discussed above namely range and quartile deviation are not measure of dispersion in the strict sense of te term because they do not show the slatterns around an average however to study the formation of a distribution we should take the deviations from an average. The two other measures namely the average deviation and the standard deviation help us in achieving this goal the mean deviation is also known as the average deviation. It is the average difference between the items in a distributing and the median or mean of that series. Theoretically there is an advantage in taking the deviations from median because the sum of deviation of item from median is minimum when signs are ignored. However in practice the arithmetic mean is more frequently used uncalculating the value of average deviation and this is the reason why it is more commonly called men deviation. In any case the average used must be clearly stated in a given problem so that any possible confusion in meaning is avoided.
Computation of mean deviation-individual observations
If X2 X1 X3 XN are N given observation then the deviation about an average A is given by
M.D. = 1/n S| X – A |
1/N S |D| or S | D| / N
Where |D| = X – A | read as mod (X-A) is the modulus value or absolute value of the deviation ignoring plus and minus signs.
Coefficient of M>D = M>D / median