Correct limits allow the user to separate probable noise from potential signals.
Of all the questions about Shewhart's charts, this is perhaps the most frequently asked question. While there is no simple answer, there are some useful guidelines. Without the follow-through of taking the right action, the computation of the right number is meaningless. Now, this is contrary to everyone's experience with arithmetic. While there are definite rules for computing limits, and right and wrong ways of computing such limits, the real power of Shewhart's charts lies in the organization's ability to use them to understand and improve their processes.