What you'll learn
How to represent grouped continuous data with histograms and related diagrams.
Histograms
- Used for continuous data; bars touch (no gaps).
- With unequal class widths, the frequency density goes on the y-axis: frequency density = frequency ÷ class width.
- Area of each bar is proportional to frequency.
Related diagrams
- Frequency polygon: plot frequency against the midpoint of each class and join with straight lines — useful for comparing distributions.
- Cumulative frequency: running total, plotted at class upper bounds; used to estimate the median and quartiles.
Exam tips
- Always use frequency density when class widths differ.
- Plot frequency polygons at midpoints, cumulative frequency at upper bounds.
Common mistakes
- Plotting frequency (not density) on an unequal-width histogram.
- Leaving gaps between histogram bars.