Statistics · Data presentation
Chapter 1 · 4
The idea
Histograms and frequency polygons
Why a histogram for continuous data plots frequency DENSITY, not frequency, so that the AREA of each bar is proportional to the frequency — and how that one rule lets you find a missing frequency, a missing class width, or read a frequency off a drawn bar. Plus the frequency polygon, which joins class midpoints.
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Statistics · Data presentation
Histograms and frequency polygons
Why a histogram for continuous data plots frequency DENSITY, not frequency, so that the AREA of each bar is proportional to the frequency — and how that one rule lets you find a missing frequency, a missing class width, or read a frequency off a drawn bar. Plus the frequency polygon, which joins class midpoints.
Why it works
The problem a bar chart can't handle
A bar chart is for categories — the bars are separate and only the height carries meaning. A histogram is for continuous data grouped into classes, and it has to cope with a problem a bar chart never faces: the classes can have different widths. If you simply plotted frequency as the height, a wide class would look far more crowded than it really is, just because its bar is fat.Area is frequency
The fix is to make area, not height, represent frequency. Each bar spans its class on the x-axis (with no gaps — the data is continuous), and its height is the frequency density:Keep reading — free
The rest of the explanation, plus 4 worked examples you step through move by move.
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