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Class intervals are grouped ranges of numerical data used to create a frequency distribution. To find class intervals, you need to determine an appropriate range of values for each interval. This is done by first finding the minimum and maximum values in the data set, then dividing the range into a number of equal intervals. For example, if the data set ranges from 0 to 10, you could divide the range into four intervals of 2.5, 0-2.5, 2.5-5, 5-7.5, and 7.5-10. Each interval is then used to assign a frequency to its corresponding data value, which can then be used to create a frequency distribution.
In a frequency distribution, a class interval represents the difference between the upper class limit and the lower class limit.
In other words, a class interval represents the width of each class in a frequency distribution.
The following examples show how to calculate class intervals for different frequency distributions.
Example 1: Calculating Class Intervals
Suppose we have the following frequency distribution that represents the number of wins by different basketball teams:
The lower class limit and upper class limit are simply the smallest and largest possible values in each class:
The class interval is the difference between the upper class limit and the lower class limit.
For example, the size of the class interval for the first class is 30 – 26 = 4.
Similarly, the size of the class interval for the second class is 31 – 35 = 4.
If we calculate the size of the class interval for each class in the frequency distribution, we’ll find that each class interval has a size of 4.
Example 2: Calculating Class Intervals
Suppose we have the following frequency distribution that represents the exam grades received by students in a certain class:
The lower class limit and upper class limit are simply the smallest and largest possible values in each class:
For example, the size of the class interval for the first class is 30 – 21 = 9.
Similarly, the size of the class interval for the second class is 40 – 31 = 9.
If we calculate the size of the class interval for each class in the frequency distribution, we’ll find that each class interval has a size of 9.