Merits and Limitation of Mean Deviation

Merits:

(i)    The outstanding advantage of the average deviation is its relative simplicity. It is simply to understand and easy to compute. Any one familiar with the concept of the average can readily appreciate the meaning of the average deviation. If a situation requires a measure of dispersion that will be presented to the general public or any group not thoroughly grounded in statistics, the average deviation is very useful.

(ii)    It is based on each and every item of the data. Consequently, change in the vale of any item would change the value of mean deviation.

(iii)    Mean deviation is less affected by the values of extreme items than the standard deviation.

(iv)    Since deviations are taken from a central value, comparison about formation of different distributions can easily be made.

Limitations:

(i)    The greatest drawback of this method is that algebraic signs are ignored while taking the deviations of the items. For example, if from twenty, fifty is deducted we write 30 and not -30. This is mathematically wrong and makes the method non – algebraic. If the signs of the deviations are not ignored the net sum of the deviations will be zero if the reference point is the mean or approximately zero if the references point is median.

(ii)    This method may not give us very accurate results. The reason is that mean deviation gives us best results when deviations are taken from median. But median is not a satisfactory measure when the degree of variability in a series is very high. And if we compute mean deviation from mean that is also not desirable because the sum of the deviations from mean (ignoring signs) is greater than the sum of the deviations from median (ignoring signs). If mean deviation is computed from mode that is also not scientific because the value of mode cannot always be determined.

(iii)    It is not capable of further algebraic treatment.

(iv)    It is rarely used in sociological studies.

Because of these limitations its use is limited and it is overshadowed as a measure of variation by the superior standard deviation.

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