The Environmental Protection Agency (EPA) tests all new cars and provides a mileage rating for both city and highway driving conditions. Thirty cars were tested and are contained in the data file Automobiles. The file contains data on several variables. In this problem, focus on the city and highway mileage data.
a. Calculate the sample mean miles per gallon (mpg) for both city and highway driving for the 30 cars. Also calculate the sample standard deviation for the two mileage variables. Do the data tend to support the premise that cars get better mileage on the highway than around town? Discuss.
b. Referring to part a, what can the EPA conclude about the relative variability between car models for highway versus city driving?
c. Assume that mileage ratings are approximately bell shaped. Approximately what proportion of cars gets at least as good mileage in city driving conditions as the mean mileage for highway driving for all cars?