A process is said to be in statistical control when the only sources of variation are from common causes. A statistically controllable process is desirable because it is predictable, while a statistically uncontrollable process will yield unpredictable distributions.
Even though a process might not be statistically controllable, it might still meet requirements. Conversely, a process which is controllable might not meet requirements. This issue is clarified by considering Process Capability.
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Published June 2018