Understanding class width is essential for anyone working with grouped data, whether you’re a student, researcher, or data analyst. The class width determines the range each class interval covers, which directly influences the accuracy of histograms, frequency tables, and statistical summaries.
What Is Class Width?
The class width (sometimes called class interval size) is the difference between the lower limit of one class and the lower limit of the next class. It tells you how wide each bin is when you group continuous data into discrete intervals.
Basic Formula
The standard formula for class width is:
Class Width = (Upper Limit of Highest Class – Lower Limit of Lowest Class) ÷ Number of Classes
After calculating, you usually round the result up to a convenient number (like 5, 10, or 20) to keep intervals tidy.
Step‑by‑Step Procedure
Step 1: Identify the Range – Subtract the smallest data value from the largest data value.
Example: If the dataset ranges from 12 to 87, the range = 87 – 12 = 75.
Step 2: Decide the Number of Classes – A common rule of thumb is Sturges’ formula: k = 1 + 3.322 log10(n), where n is the total number of observations.
Step 3: Compute the Preliminary Width – Divide the range by the number of classes.
Continuing the example: If n = 50, then k ≈ 1 + 3.322 log10(50) ≈ 6.4 → round to 7 classes.
Preliminary Width = 75 ÷ 7 ≈ 10.7.
Step 4: Round Up – Choose a round number equal to or larger than the preliminary width. Here, you might select **11
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