A firm tested the durability of four different types of tires by having 4 Cadillacs each randomly equipped with one of the types of tires. In similar fashion, 4 Fords were used, each randomly equipped with one of the tire types. In a like manner, 4 Volkswagens were each randomly outfitted with one of the four tire brands. Then each of the twelve cars ran over a test track until the tires showed signs of serious wear. At that point the average mileage of the three cars using one type of tire was compared to the average mileage of the three cars using the second type of tires, and so on. This design is: