For example, we are given some mid-values as 72, 77, 82, 87, .... Now, consider the first mid-value 72 and also the differences between successive mid-values.
We have 77 - 72 = 5, 82 - 77 = 5, 87 - 82 = 5 ....
which gives the class-length i = 5.
For the first class-interval,
l1 = m - i/2
= 72 - 5/2 = 69.5
= 72 + 5/2 = 74.5.
Thus the first class-interval is 69.5 - 74.5
and other class-intervals then are 74.5 - 79.5, 79.5 -
84.5, 84.5 - 89.5
Open-end Class Intervals : In any question when the lower limit of the first class-inteval or the upper limit of the last class-interval, are not given then subtract the class length of the next immediate class-interval from the upper limit. This will give us the lower limit of the first class-interval. Similarly add the same class length to the lower limit of the last class-interval. 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