An experiment is to be conducted to compare four different brands of gasoline for gas mileage.

a. Suppose that you randomly divide 50 cars into five groups of 10 cars each and then randomly assign the five groups to the five brands of gasoline, one group per brand. Is this experimental design a completely randomized design or a randomized block design? If it is the latter, what are the blocks?

b. Suppose, instead, that you use ten different models of cars whose varying characteristics (e.g., weight and horsepower) affect gas mileage. Five cars of each model are randomly assigned to the five different brands of gasoline. Is this experimental design a completely randomized design or a randomized block design? If it is the latter, what are the blocks?

c. Which design is better, the one in part

(a) or the one in part (b)?

Explain your answer.