Why is SPC (Statistical Process Control) Important?

[@nationalparks-f8q]

How is the process mean in the control chart determined? Is provided in the design requirements? Or is it the mean value of the data points, but if so, how do you know if that mean is is appropriate?

[@brucema5659]

All these years in manufacturing, no one has ever explain why Test 6 is 4 out of 5 points outside of one sigma, not 4 out of 4 or 5 out of 5. I even looked into the WE book and it wasn’t explained. So strange.