You are interested in determining which of two brands of windshield wipers, brand A and brand B, lasts longer under differing conditions of use. One hundred Toyota Highlanders are fitted with brand A wipers and 100 Chevrolet Equinoxes are fitted with brand B wipers. Each pair of wipers is used for two years. Wiper wear is then measured for each pair of wipers, and the average windshield wiper wear for the two brands is compared. What is wrong with this experimental design?