A tire manufacturer wishes to investigate the tread life of

A tire manufacturer wishes to investigate the tread life of its tires. A sample of 10 tires driven 50,000 miles revealed a sample mean of 0.32 inch of tread remaining with a standard deviation of 0.09 inch. To construct a 95 percent Confidence Interval for the population mean, you will need 97.5-percentile of a ____.

a. t distribution with degree of freedom 9.

b. t distribution with degree of freedom 10.

c. Standard normal distribution (z distribution).

Solution

Since we do not have the population standard deviatoin we must use t-distribution with n-1 degrees of freedom.

so the correct option is (a). t distribution with degree of freedom 9.