React Control Chart
Is my process stable? — a different question from is this part in spec?, and the one that catches trouble before scrap does. This is an individuals chart with shaded sigma zones and automatic Western Electric rule detection, so a drift is flagged while it is still a drift.
Etch depth by lot
x̄ 249 nm · σ 1.26 nm · UCL 253 · LCL 245
- 1Point beyond 3σ
- 48 consecutive on one side of centre
Installation
Props
| Prop | Type | Description |
|---|---|---|
| data | ControlPoint[] | { label, value } in sample order |
| title | string | Chart heading |
| unit | string | Appended to statistics, e.g. " nm" |
| centerLine | number | Overrides the computed process mean |
| sigma | number | Overrides sigma from the average moving range |
| usl lsl | number | Spec limits — drawn distinctly from control limits |
| rules | boolean | Western Electric detection (default true) |
| height | — | SVG height (default ) |
Sigma from the moving range, not the sample
The one subtlety that separates a real control chart from a line graph with lines on it: sigma is estimated from the average moving range (mR̄ / d₂, with d₂ = 1.128 for consecutive pairs), not from the sample standard deviation.
If you used the sample standard deviation, a genuine special cause — the exact thing the chart exists to detect — would inflate the estimate and pull the control limits out to swallow itself. The moving-range estimate reflects only point-to-point variation, so an out-of-control run still lands outside the limits.
The rules
Point markers are labelled with every rule they trip:
| Rule | Signal |
|---|---|
| 1 | One point beyond 3σ |
| 2 | 2 of 3 consecutive beyond 2σ, same side |
| 3 | 4 of 5 consecutive beyond 1σ, same side |
| 4 | 8 consecutive on one side of the centre line |
The demo shows both classic failure modes: a single excursion at L-113 (rule 1) and a slow upward drift over the final lots (rule 4) that never breaches a limit but is unmistakably not random. Spec limits (usl/lsl) are drawn in a distinct dashed style, because "in control" and "in spec" are independent — a process can be perfectly stable and still centred too close to a limit.
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