The manager at a new tire and repair shop in Hartford, Connecticut, wants to establish guidelines for the time it should take to put new brakes on vehicles. In particular, he is interested in estimating the mean installation time for brakes for passenger cars and SUVs made by three different manufacturers.

To help with this process, he set up an experiment in his shop in which five brake jobs were performed for each manufacturer and each class of vehicle. He recorded the number of minutes that it took to complete the jobs. These data are in a file called Brake-test.

a. Use Excel to compute the point estimate for the population mean installation time for each category.

b. Use Excel to compute the necessary sample statistics needed to construct 95% confidence interval estimates for the population mean installation times. What assumption is required?

c. Based on the results from part

b, what conclusions might you reach about the three companies in terms of the time it takes to install their brakes for passenger cars and for SUVs? Discuss.