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What is a Class Interval? - Definition & Example

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  • 0:04 What Is a Class Interval?
  • 1:33 An Example Using Class…
  • 3:21 Lesson Summary
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Lesson Transcript
Instructor: Jenny Homer

Jenny has masters' degrees in public health and public administration.

This lesson introduces class intervals, a way to divide data into groups. Learn when to use class intervals and how they can be helpful, using the interesting example of the U.S. Census.

What Is a Class Interval?

Ever wonder exactly how many people live in your state? Or in the whole country? The United States government takes a census, or a count of the number of people living in the country. In 2010, the population of the United States was 308.7 million people. Taking a census is such a big job that it's done only once every ten years.

The U.S. Census also collects information about people, like how old they are and how long they have gone to school. The census is collecting data, numbers that give information.

Think about the question, 'How old are you?' There are a lot of different answers you could give, from 1 if you're a very little kid all the way past 100 if you're very old. But knowing exactly how many 1-year-old kids, how many 2-year-old kids, how many 3-year-old kids, and so on is not always the easiest or best way to look at the data. That is where it's helpful to use a class interval. A class interval is a way to divide data and group certain answers together.

When determining the class interval to use with your data, you must follow three rules:

  1. The same person or unit can be in only one class interval.
  2. The width or range of numbers in the class intervals must be equal.
  3. There are no numbers left out of the groupings.

And, although not a rule, you typically want to have 10 to 20 class intervals.

An Example Using Class Intervals

Let's find out what this means for the U.S. Census. Individually listing the number of 1-year-old kids, 2-year-old kids, 3-year-old kids, and so on, could be too much. It would make more sense to group the ages into class intervals.

Do class intervals work here? Let's look at our rules:

Rule #1 says the same person or unit can be in only one class interval. Can a person be 1 and 10 years old at the same time? Of course not! You can only be one age at a time, so the same person can only be in one class interval at a time.

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