n = 4 values are sampled every hour from a process that under "all OK stable process" conditions produces observations x that are normal with mean 20 and standard deviation 4. A typical Shewhart x chart is set up.
(a) What is the all OK ARL of the monitoring scheme?
(b) An upward shift in the process mean of at least 1 unit occurs, while the process standard deviation variation does not change. At worst, how many hours on average will pass before this change produces a subgroup average outside the control limits?