A survey reports the percentage of households owning each number of vehicles as — 0 vehicles: 10 percent, 1 vehicle: 25 percent, 2 vehicles: 40 percent, 3 vehicles: 20 percent, 4 vehicles: 5 percent. What is the mean number of vehicles per household?

Percentages can be used as frequencies directly, since they are frequencies out of a group of 100 households. So Σfx = 0(10) + 1(25) + 2(40) + 3(20) + 4(5) = 0 + 25 + 80 + 60 + 20 = 185, and the mean is 185 ÷ 100 = 1.85 vehicles. The value 37 divides 185 by 5, the number of CATEGORIES, instead of by the 100 households the percentages describe — a reliable sign of this error is an answer far outside the range of the data, since no household owns more than 4 vehicles. The value 18.5 divides by 10, a place-value slip. The value 2 is the modal number of vehicles, owned by the largest single group of households at 40 percent, which answers a different question about the same table.