There are very few times when you should start to investigate a process once you have established Statistical Process control (SPC), and the process is considered to be under control. What is the main reason you should investigate?
a. Behavior is normal
b. A single plot falls outside the control limits
c. A large number of plots are on or near the center line
d. 3 out of six values are above the central line
e.There is no real trend in any direction