Specification and Control Limits
Specification and control limits
Specification limits are the permitted boundaries required for a product; control limits are boundaries based on process data variation for identifying abnormal signals.
In context
The two boundaries serve different purposes. Even if all products are within specifications, a continually shifting process mean requires investigation of the change.
Example
Hypothetical length measurements may all be within tolerance yet shift in one direction after a particular time. Acceptance and process stability are different questions.
Key distinction
Do not use control limits as customer acceptance ranges or arbitrarily change specification limits to fit the data.
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