Range & IQR

Two classes both average 75 on a test. But Class A scored 73, 74, 75, 76, 77 — tight as a drum. Class B scored 40, 60, 75, 90, 110 — all over the map. The mean alone calls them identical; the spread tells the truth. A center without a spread is half a description. You need to know how scattered the data is, and the first spread you'll meet is the range — and its stronger cousin, the IQR.