How to Find a Class Interval

How to Find a Class Interval
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  • 0:03 What Are Class Intervals?
  • 1:21 Organizing Data into a…
  • 2:24 Lesson Summary
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Lesson Transcript
Instructor: Carrie Buscher

Carrie has a master's degree in learning disabilities and has taught elementary math for over 20 years.

There's a lot of data out there! We often use tables and histograms to help us understand data. In this lesson, you will learn how to find a class interval to make data more useful.

What Are Class Intervals?

Imagine your class has grown crystals as a science experiment. Each of the twenty crystals has been weighed, and now you want to organize the data. The weight of each crystal in grams can be seen below:

19, 23, 24, 24, 28, 28, 28, 29, 30, 32, 32, 33, 34, 34, 35, 36, 37, 39, 40, 44

When determining a class interval, which is the range of the data for each group, the first thing you need to decide is how many groups, or classes, of data you want. Let's say you decide on 6 classes for the data. The next step is to find the difference between the heaviest and lightest weights, which you may have already figured out is 25 grams (44 - 19 = 25).

Divide the difference (in this case 25) by 6 (remember that this is the number of classes that you wanted to use) to get 4.16. Now, we always want our class interval to be a whole number, so in this case, it makes sense to use a class interval of 4 or 5. Here's what the frequency table, a table that summarizes how often (frequently) certain numbers appear, will look like if you use a class interval of 5:

Weight (in grams) Frequency
19 - 23 2
24 - 28 5
29 - 33 5
34 - 38 4
39 - 43 2
44 - 48 1

As you can see, each class of data has 5 weights, such as 19, 20, 21, 22, 23. Because the data is organized, it's easier for you to analyze it, such as concluding that most of the crystals (14 out of 20) weigh between 24 and 38 grams.

Organizing Data into a Histogram

Your class also wanted to compare the heights of the crystals. The heights of each crystal (in millimeters) is shown below:

33, 33, 34, 40, 41, 41, 42, 43, 45, 46, 47, 47, 47, 48, 50, 51, 55, 56, 58, 60

Again, we find the class interval by finding the difference between the tallest and shortest crystals and dividing that number by the number of classes wanted. So:

  • 60 - 33 = 27
  • 27 / 5 = 5.4

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