A machine produces metal rods used in an automobile suspension system. A random sample of 15 rods is selected and the diameter is measured. The resulting data (in millimeters) are as follows: No. 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15
Dia. 8.21 8.21 8.23 8.25 8.26 8.23 8.20 8.26 8.19 8.23 8.20 8.28 8.24 8.25 8.24

Use the data above to calculate a 95% two-sided confidence interval on the mean rod diameter. Assume the data are normally distributed.

a. Calculate the sample mean and standard deviation.
b. Calculate the 95% two-sided confidence interval on the true mean rod diameter.