Trees in a park are grouped by height in metres as 0–10: 20 trees, 10–30: 32 trees, 30–60: 36 trees, 60–100: 40 trees. Which class is the most crowded, that is, which has the greatest number of trees per metre of height?

The classes have different widths, so raw frequencies cannot be compared with one another — each must be divided by the width of its own class. The densities are 20 ÷ 10 = 2.0, 32 ÷ 20 = 1.6, 36 ÷ 30 = 1.2 and 40 ÷ 40 = 1.0 trees per metre, so 0–10 is the most crowded even though it holds the fewest trees. The class 60–100 carries the largest count but spreads it over 40 metres. This is exactly why a histogram with unequal classes must be drawn with heights proportional to density rather than to frequency.