Consider a manufacturing process that is producing hypodermic needles that will be used for blood donations. These needles need to have a diameter of 1.65 mm—too big and they would hurt the donor (even more than usual), too small and they would rupture the red blood cells, rendering the donated blood useless. Thus, the manufacturing process would have to be closely monitored to detect any significant departures from the desired diameter. During every shift , quality control personnel take a random sample of several needles and measure their diameters. If they discover a problem, they will stop the manufacturing process until it is corrected. Suppose the most recent random sample of 35 needles have an average diameter of 1.64 mm and a standard deviation of 0.07 mm. Also, suppose the diameters of needles produced by this manufacturing process have a bellshaped distribution.

a. Identify the observational unit for this study.
b. Identify the variable of interest and whether it is categorical or quantitative.
c. Use the 2SD approach to find a 95% confidence interval for the average diameter of needles produced by this manufacturing process.