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•Stable Processes:
•In theory stable processes have only common causes
•This mean no trends or process shifts
•Stable processes can still have large variation compared with the
specifications limits
•Unstable Processes:
•Common causes are always present
•Special causes can occur at certain times only causing trends and process
shifts
•The example above shows a process that starts as a stable one, then a
special cause take effect and a trend starts