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It is a location based measure of dispersion and is defined as follows:

where Q1 is lower quartile and Q3 is upper quartile. This Q.D. is also known as semi inter- uartile range, whereas the quantity Q3 - Q1 is called inter-quartile range.

Note. I. For comparative studies of variability of two distributions a relative measure is given as

follows :

Coefficient of    

2. If there is two much variability between two distributions then the following measure is  calculated :

3. In the case of unequal class-intervals Q.D. is sometimes preferred to S.D.