Suppose a researcher would like to determine whether one grade of gasoline produces better gas mileage than another grade. Twenty cars are randomly divided into two groups, with 10 cars receiving one grade and 10 receiving the other. After many trips, average mileage is computed for each car.

a. Would it be easier to detect a difference in gas mileage for the two grades if the 20 cars were all the same size, or would it be easier if they covered a wide range of sizes and weights? Explain.

b. What would be one disadvantage to using cars that were all the same size?