Over several weeks of normal, or in-control, operation, 20 samples of 150 packages each of synthetic-gut tennis strings were tested for breaking strength. A total of 141 packages of the 3000 tested failed to conform to the manufacturer's specifications.
a. What is an estimate of the process proportion defective when the system is in control?
b. Compute the upper and lower control limits for a p chart.
c. With the results of part (b), what conclusion should be made about the process if tests on a new sample of 150 packages find 12 defective? Do there appear to be assignable causes in this situation?
d. Compute the upper and lower control limits for an np chart.
e. Answer part (c) using the results of part (d).
f. Which control chart would be preferred in this situation? Explain.