The heights of 60 saplings in a nursery, in centimetres, are grouped as 20–35: 8, 35–50: 10, 50–65: 20, 65–80: 16, 80–95: 6 (class interval: frequency). Find the median height in cm.

Here n = 60, so n/2 = 30. The cumulative frequencies are 8, 18, 38, 54, 60, and the first to reach 30 is 38, so the median class is 50–65 with l = 50, cf = 18, f = 20 and h = 15. Median = 50 + ((30 − 18)/20) × 15 = 50 + 9 = 59. The class mark 57.5 is the middle of the median CLASS, not the middle of the data; 50 is only the lower boundary of that class; and 44 comes from using the median class's own cumulative frequency 38 in place of the preceding 18, which pushes the answer backwards out of the class altogether — a sure sign that the wrong figure has been used.