Dr. Goodrich wanted to demonstrate that his tires were better than those of his competitor, Dr. Goodyear. From car registration and leasing records, he found 40 salespeople who drove the same model of automobile approximately the same number of miles per week. Anonymously, Dr. Goodrich hired an independent research assistant, who was unaware of the purpose of the study, to randomly assign to 20 of the sales people a new set of unmarked Goodrich tires, and to the other 20 a new set of unmarked Goodyear tires of the same price and quality. After six months and an average of 15,000 miles traveled by both groups, the assistant arranged for the salespeople to exchange tires. After another six months, and similar mileage, the assistant measured the amount of tread wear and reported that the Goodrich tires had actually worn more than the Goodyear tires.
(a) Identify the independent variable(s).
(b) Identify the dependent variable(s).
(c) Identify any confounding variable(s).