Any process has a normal operational range within which the process is said to be under control.
The limits to this range are typically divided into alert and action and acceptance limits.
Alert Limits
As long as the process is within the alert limits, the process or product is said to be under control and will meet its intended requirements.
Once a process exceeds the alert limits, this should raise concerns—unless, for example, there was a known event which would cause a drift in the process or product. An action is not necessarily required, but it is a flag to pay attention and monitor this specific process or parameter.
Action Limits
The next level is the action limit. Once a process or product exceeds the action limit an investigation into the excursion must be prompted and a corresponding corrective action implemented if possible.
It should be noted that a corrective action to an action limit excursion is not typically documented as a formal CAPA (Corrective and Preventative Action), but is managed within an SPC process.
If the action limit is set appropriately, product or processes which were manufactured over the
action level should still be capable of meeting its intended requirements and therefore should be conforming to its specifications.
Specification
The last level of action is the specifications, or acceptance limit. If the product exceeds the acceptance limit or specification requirements, then the product is nonconforming and must be contained and a full non-conformance investigation and disposition process takes over.
Trending & Monitoring
By trending and monitoring critical components, product parameters and process controls any changes to the product quality can be identified early and addressed prior to it becoming a nonconforming product or product which is not suitable for its intended use, provided the right parameters and limits have been identified.
This is truly the key to controlling the quality of the product.