For example, we are given some midvalues as 72, 77, 82, 87, .... Now, consider the first midvalue 72 and also the differences between successive midvalues.
We have 77  72 = 5, 82  77 = 5, 87  82 = 5 ....
which gives the classlength i = 5.
For the first classinterval,
l1 = m  i/_{2}
= 72  5/_{2} = 69.5
and l2
= 72 + 5/_{2} = 74.5.
Thus the first classinterval is 69.5  74.5
and other classintervals then are 74.5  79.5, 79.5 
84.5, 84.5  89.5
Openend Class Intervals : In any question when the lower limit of the first classinteval or the upper limit of the last classinterval, are not given then subtract the class length of the next immediate classinterval from the upper limit. This will give us the lower limit of the first classinterval. Similarly add the same class length to the lower limit of the last classinterval. But always notice that the lower limit of the first class ( i.e. the lowest class) must not be negative or less than 0. For example :
Table  3
