When dealing with a large set of data values it is convenient to divide the data into intervals of equal length called class intervals. The class intervals formed are of two types – Inclusive Class Intervals and Exclusive Class Intervals.

**Inclusive Class Intervals:**

By the intervals being inclusive we mean that both the upper and lower limits of the interval are included in the interval. So in the example below, we have a frequency of 3 for the interval 30-39. This means that there a three data points whose value lies between 30 and 39 including 30 and 39 themselves. For instance, the three values could be 30, 34, and 39.

**Exclusive Class Intervals:**

Generally speaking, in exclusive class intervals upper limit of the interval is not considered to be a part of the values covered by that interval. For example, in the table below the interval, 15-20 has a frequency of 5. This means that there are 5 data points taking values between 15 and 20 where 15 is allowed but 20 is excluded. For instance, the 5 data points 15, 16, 17, 17, and 18 would be considered to belong to this interval. On the other hand, the data values 15, 16, 17, 18, and 20 would not be considered to be part of this interval since 20 is excluded from this interval. The value 20 can belong to the next class interval which is 20-25.