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Class Width: Simple Definition
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A frequency distribution table
showing a class width of 7 for
IQ scores (e.g. 125-118 = 7)
Class width refers to the difference between the upper and lower boundaries of any class (category). Depending on
the author, it’s also sometimes used more speci cally to mean:
The difference between the upper limits of two consecutive (neighboring) classes, or
The difference between the lower limits of two consecutive classes.
Note that these are different than the difference between the upper and lower limits of a class.
Calculating Class Width in a Frequency Distribution
Table
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Class Width for a Frequency Distribution Table
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In a frequency distribution table, classes must all be the same width. This makes it relatively easy to calculate the
class width, as you’re only dealing with a single width (as opposed to varying ones). To nd the width:
1. Calculate the range of the entire data set by subtracting the lowest point from the highest,
2. Divide it by the number of classes.
3. Round this number up (usually, to the nearest whole number).
Example of Calculating Class Width
Suppose you are analyzing data from a nal exam given at the end of a statistics course. The number of classes you
divide them into is somewhat arbitrary, but there are a few things to keep in mind:
Make few enough categories so that you have more than one item in each category.
Choose a number that is easy to manipulate; usually, something between ve and twenty is a good idea. For
example, if you are analyzing a relatively small class of 25 students, you might decide to create a frequency
table with ve classes.
Example: Find a reasonable class with for the following set of student scores:
52, 82, 86, 83, 56, 98, 71, 91, 75, 88, 69, 78, 64, 74, 81, 83, 77, 90, 85, 64, 79, 71, 64, and 83.
1. Find the range by subtracting the lowest point from the highest: the difference between the highest and
lowest score: 98 – 52 = 46.
2. Divide it by the number of classes: 46/5, = 9.2.
3. Round this number up: 9.2≅ 10.
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References
Gleaton, James U. Lecture Handout: Organizing and Summarizing Data. Retrieved from
http://www.unf.edu/~jgleaton/LectureTransCh2.doc on August 27, 2018.
Gonick, L. (1993). The Cartoon Guide to Statistics. HarperPerennial.
Jones, James. Statistics: Frequency Distributions & Graphs. Retrieved from
https://people.richland.edu/james/lecture/m170/ch02-def.html on August 27, 2018.
Levine, D. (2014). Even You Can Learn Statistics and Analytics: An Easy to Understand Guide to Statistics and
Analytics 3rd Edition. Pearson FT Press
CITE THIS AS:
Stephanie Glen. "Class Width: Simple De nition" From StatisticsHowTo.com: Elementary Statistics for the
rest of us! https://www.statisticshowto.com/probability-and-statistics/descriptive-statistics/frequencydistribution-table/class-width/
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Probability and Statistics Topic Indexes
Basic Statistics.
Bayesian Statistics and Probability
Descriptive Statistics: Charts, Graphs and Plots.
Probability.
Binomial Theorem.
De nitions for Common Statistics Terms.
Critical Values.
Hypothesis Testing.
Normal Distributions.
T-Distributions.
Central Limit Theorem.
Con dence Intervals.
Chebyshev's Theorem.
Sampling and Finding Sample Sizes.
Chi Square.
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Online Tables (z-table, chi-square, t-dist etc.).
Regression Analysis / Linear Regression.
Non Normal Distributions.
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