SPC Control Charts and Capability
Statistical process control turns the readings your inspectors take into a picture of whether a process is steady and whether it is capable of holding the specification. It uses the measured readings already captured on inspections, so nothing extra has to be entered — if your inspection plans ask for measurements, the charts fill themselves.
The control chart
Choose an item with measured readings and the tab draws two charts one above the other:
| Chart | What it shows |
|---|---|
| Individuals | Each reading in sequence against the process mean and the control limits. |
| Moving Range | The difference between one reading and the next — how much the process is jumping about. |
Points are plotted per subgroup, and the chart states how many subgroups and readings it is built from. At least two inspections are needed before a control chart can be drawn; below that the tab says so rather than drawing a line through one point.
Capability: Cp and Cpk
Control limits tell you whether the process is behaving consistently. Capability tells you whether consistent is good enough for the specification you have to meet.
| Cpk | Reading |
|---|---|
| 1.33 or more | Capable. The process comfortably fits inside its limits. |
| 1.0 to 1.33 | Marginal. It fits, but with little room. Drift will start producing failures. |
| Below 1.0 | Not capable. The process cannot reliably hold the specification. |
Capability needs a specification limit to measure against. A characteristic with no limit set is marked Needs a specification limit — add it on the inspection plan and the figure appears.
The capability summary
All items: capability summary lists every item with measured readings in the period and its capability figure, so you can see which processes need attention without opening each chart in turn. The same figures print through the Process Capability report — see Quality reports.
No readings means no charts. If the tab says No measured readings in the period, the item's inspection plan is recording attributes — pass or fail — rather than measurements. SPC needs numbers.
Who can see it
SPC and the supplier scorecards share the Quality Reports permission. It can be granted on its own, which suits a manager or an auditor who should read the analysis without being able to inspect, decide or disposition anything.