Before you understand this concept, you are advised to read:
What is Frequency Distribution Table ?
As you know that data organized in the form of frequency distribution table helps us to read it easily. And also we could drive answers from it more quickly.
But sometimes the raw data received is very long and lengthy that if we prepare a frequency distribution table, the table will be very long and also it is inconvenient to draw result from such a long table.
Example : In a class of 30 students, following numbers represents weight of these students:
30, 31, 51, 35, 40, 39, 42, 43, 36, 49, 44, 33, 37, 47, 52, 38, 32, 50, 39, 40, 45, 46, 53, 35, 31, 41, 51, 33, 46, 53
Now, you can observe that if we prepare a simple Frequency Distribution Table, it will be very long and it will inconvenient to find answer from this. So, for this we put the data in groups and make a Grouped Frequency Distribution Table
Groups (Weight in Kg)  Frequency (No. of Students) 

30 – 35  6  35 – 40  7  40 – 45  6  45 – 50  5  50 – 55  6  Total  30 
Hence, from the above grouped frequency distribution table we can conclude that maximum students comes in weight category of 35 – 40
The Groups i.e. 3035, 3540, 4045, 4550, 5055 all are known as Class Intervals
In every Class interval, there is one Lower Class Limit and one Upper Class Limit i.e.
In 30 – 35: 30 is lower class limit and 35 is upper class limit
In 35 – 40: 35 is lower class limit and 40 is upper class limit
In 40 – 45: 40 is lower class limit and 45 is upper class limit
In 45 – 50: 45 is lower class limit and 50 is upper class limit
In 50 – 55: 50 is lower class limit and 55 is upper class limit
The difference between the Upper Class Limit and Lower Class Limit is called width or size of Class Intervals. We must remember that width of Class Intervals should be same in the entire table. As you can observe in the above example that the Width of Class Interval is same, i.e. 5 in each case.
Also, you will observe that few observations are same among different class intervals i.e.
35 is present in two Class intervals (3035 & 3540)
40 is present in two Class intervals (3540 & 4045)
45 is present in two Class intervals (4045 & 4550)
50 is present in two Class intervals (4550 & 5055)
So, will preparing the table, we cannot put entry of common observation in two class interval. Doing this leads to incorrect results.
Hence, common observation entry is done in higher class interval only, i.e.
35 is to be entered in class interval of 3540 and not in 3035
40 is to be entered in class interval of 4045 and not in 3540
45 is to be entered in class interval of 4550 and not in 4045
50 is to be entered in class interval of 5055 and not in 4550

