Introduction
We have already learnt in detail the measures of central tendency  namely mean, median and mode of given data. These measures give a single number as the representative of the whole data. But they do not tell us how the observations are scattered about the average. For example, two distributions giving weekly wages of 200 persons may have the same mean, say, Rs100. In one distribution most of the observations may be cented around the mean value 100; a few others may be away from 100. In an another distribution, a large number of observations may be above 150 and another set of large number of observations may be below 50 and only a few between 50 and 100 and still the mean may be 100.
This fact may be illustrated by a diagram as shown below.
Wages  No. of Workers  
25  5  
30  7  
50  13  
75  12  
100  15  
125  20  
150  15  
165  7  
175  6 
Wages  No. of Workers 

80  5  
90  10  
100  50  
110  20  
120  10 
The various measures of dispersion or variability are
 Range
 Quartile Deviation
 Mean Deviation
 Standard deviation.