What is it
A histogram displays continuous grouped data using bars whose area (not height) represents frequency — requiring the use of frequency density (frequency ÷ class width) on the vertical axis.
Why it matters
Histograms are commonly confused with bar charts, but the area-represents-frequency rule makes them a genuinely different (and more mathematically precise) way of representing data with unequal class widths.
Exam tip
Always calculate frequency density as frequency ÷ class width before plotting or reading a histogram — plotting frequency directly instead of frequency density is the most common histogram mistake.
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