We reconsider SteelCo from the previous two problems from a different point of view. Now we assume SteelCo doesn't know its process mean and standard deviation, so it uses sampling. The file P20_64.xlsx lists 150 randomly sampled rod lengths.
a. Calculate a 95% confidence interval for the population mean length of all rods produced.
b. Continuing part a, find a 95% confidence interval for the population proportion that meet the specifications listed at the top of the spreadsheet (the same as in the previous problem).
c. Using the sample standard deviation found in part a as a best guess for the population standard deviation, how large a sample size is required to achieve a 95% confidence interval for the mean of the form "point estimate plus or minus 0.002"?