The largest value of ‘x’ is B and smaller value is A. then x = B – A is the total range of ‘x’. A large range indicates that the total the values of ‘x’ are spread over a large interval or the variation of values of ‘x’ is large. A small range indicates smaller variation in the values of ‘x’, this, the range is measures of variation (or dispersion) of ‘x’.
In order to get a better idea about the distibution of values within the range, we should subdivide the total range into a number of class intervals and find out the number of values in different classes. The main points underlying the construction of a frequency distribution are as follows.
a. Construction of Discrete frequency Distribution:
1. Prepare a table with three columns first for variable under study, second for ‘Tally bars’ and the third for the total, Representing corresponding frequency to each value of size of the variable.
2. Place all the values of the first column ascending order – beginning with the lowest end giving to the highest. The gap between one magnitude to another may preferably be the same.
3. Put bars (vertical lines) in front of the values according in the second column keeping in view the number of items a particular value repeats, itself. This column is for facility in counting blocks of five bars or 
are prepared and some space is left between each block of bars.
4. Count the number of bars in respect of each value in the variable and place it in the third column made for total of frequency.
b. Construction of continuous frequency Distribution:
There are two methods of classifying the data according to class intervals.
Exclusive method:
Under this method upper limits are excluded. The upper limits of class intervals is the lower limit of the next class for example, if the students obtained marks are grouped as 5 – 10, 10-15, 15-20, 20-25, 25-30 etc. we include in first group of students whose marks are 5 or more but under 10. If the marks of a students are 10 he is not included in the first group but in the second, i.e., 10 -15
Inclusive method:
Under this method upper class limits of classes are included in respective classes for example, if the students obtained marks are grouped as 5 – 9, 10-14, 15-19, 20-24, 25-29 etc in the group 5-9, we include in first group students whose marks is between 5 and 9. If the marks of students are 10 he is included in the next class, i.e., 10 to 14. If there are no whole numbers, the classes can be made 5-9.9, 10-14.9, 15-19.9 and so on.
