I'm super confused as to how do I know which formula to use for calculating the UCL and LCL? For #19, I had to use the p-bar formula. But for #20, how come I don't use the p-bar formula and instead I'm calculating mean? I know how to calculate these problems, I just don't get how do I know when to use a certain formula.

20 Problem 10-7 (Algo) 1.9 points The postmaster of a small western town receives a certain number of complaints each day about mail delivery. Skipped 1 4 Number of complaints 2 11 3 14 4 8 5 9 DAY 7 8 5 13 6 6 9 15 10 7 11 6 12 4 13 2 14 11 Click here for the Excel Data File eBook a. Determine three-sigma control limits using the above data. (Round your intermediate calculations to 4 decimal places and final answers to 3 decimal places. Leave no cells blank - be certain to enter "O" wherever required. Round up any negative control limit value to zero.) Hint Print UCL LCL References b. Is the process in control? Yes O No 19 Problem 10-6 (Static) .9 points A medical facility does MRIs for sports injuries. Occasionally, a test yields inconclusive results and must be repeated. Using the following sample data and n = 200. eBook 1 1 2 2 Number of retests Nw 3 2 4 0 NU 5 2 SAMPLE 6 7 8 1 2 0 PO NY 9 2 NO 10 11 12 7 3 2 Hint Print References Click here for the Excel Data File a. Determine the upper and lower control limits for the fraction of retests using two-sigma limits. (Do not round intermediate calculations. Round your final answers to 4 decimal places. Leave no cells blank - be certain to enter "O" wherever required.) UCL 0.0234 LCL 0