A capability summary needs a defensible process basis. Keep time order so drift and changes remain visible.
Why time behavior comes first
Process stability concerns a consistent distribution over time, including its location and variation. A capability summary compares process behavior with specification limits. If the data combine changing operating conditions, the calculated summary can describe that mixture rather than one stable process. A time-ordered plot is therefore a necessary discussion tool before a capability interpretation.
A hypothetical apparently acceptable average
Imagine early observations clustered below a target and later observations above it. Their overall average sits near the target, but the time plot exposes a directional change. Sending only the mean and one index removes the evidence that the trial condition evolved.
Keep the original sequence and mark genuine events such as an agreed recipe change or product transition. Do not remove inconvenient points until a documented reason and analysis rule support exclusion. This illustration contains no actual HM readings and does not establish a universally sufficient run duration.
Define the process being summarized
Identify product, pack, machine configuration, measurement method and operating condition. Ask whether start-up, intentional changeover and steady operation belong in separate analyses. Preserve those categories as recorded rather than choosing boundaries after seeing which result looks best.
When requesting a supplier review, send the ordered data and event annotations with the capability question. Discuss whether enough representative operation has been observed for the proposed interpretation. Stability assessment and product conformity remain distinct decisions; a stable process can still operate outside the buyer’s requirement, and an acceptable short sample does not prove long-run stability.
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 |
|---|---|
| What process condition is represented? | Defined product, pack and configuration |
| Can the readings be plotted in time order? | Original sequence and event labels |
| Were any exclusions made? | Reason and agreed analysis rule |
Customer Questions
Does an average on target prove stability?
No. Opposing changes can cancel in an overall average.
Does stability prove conformance?
No. A stable distribution can still conflict with specification limits.
Should intentional changes be hidden in one index?
Their treatment should follow the stated analysis question and recorded conditions.
Primary References
These references support the technical principles discussed in this guide. The worked examples and review questions are educational.
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