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A frequency distribution groups data into classes of equal width and counts how many observations fall in each. The class width is the range divided by the number of classes, and the modal class holds the most observations.
Class width
width = (maximum − minimum) / number of classes
Sturges' rule suggests about 1 + log₂n, so roughly 4 or 5 classes for a dozen observations.
Here each class is half-open [lower, upper), with the top class closed so the maximum is included.