A control-chart signal and an out-of-specification item answer different questions.
Name each boundary by its purpose
Control limits support assessment of whether the monitored process behaves consistently with its statistical basis. Specification limits describe an approved requirement for the characteristic. Their origins and purposes differ. Product observations can satisfy a specification while a chart indicates a changed process, or a statistically consistent process can produce outcomes outside the requirement.
A hypothetical changed but conforming observation
Imagine a reading inside the approved product limits that falls beyond a monitoring limit calculated from the established baseline. The first statement concerns product conformity; the second prompts investigation of process consistency. Neither statement should erase the other.
Conversely, stable operation centred poorly relative to the product requirement can remain unacceptable. These examples do not prescribe control limits, product tolerances or a release decision for HM equipment. They show why two reference fields are needed.
Keep monitoring and disposition traceable
Attach the specification reference and revision separately from the chart method, baseline dataset and control-limit version. Record the plotted characteristic, unit and sampling structure. Agree who responds to a chart signal and who owns product disposition.
Do not recalculate a limit merely to make the latest reading fit. An approved baseline review may be appropriate after a genuine process change, but preserve the change rationale and earlier basis. When discussing a filling or inspection trial, send both boundaries with their names so a supplier can understand which concern the buyer is raising.
Trial discussion worksheet
This blank worksheet is for your own project. It contains no H M machine trial result.
| Review question | Reference or observation to retain |
|---|---|
| Where did specification limits come from? | Approved requirement reference |
| Where did chart limits come from? | Baseline and method |
| Which decision is open? | Process investigation or product disposition |
Customer Questions
Is a control limit a product tolerance?
No. Its monitoring purpose and statistical basis differ.
Can a conforming reading trigger a process investigation?
Yes. It may differ from the established monitoring pattern.
Should chart limits be changed for each inconvenient reading?
A baseline change needs its own justified and documented basis.
Primary References
These references support the technical principles discussed in this guide. The worked examples and review questions are educational.
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