# Read my SPC chart and process capability results

A plain-English read of a control chart and Cp/Cpk results: whether the process is stable, capable, and what to do next.

- Topic: Data & Analysis
- Framework: Few-Shot
- Level: Advanced
- Tags: Fill-in Template
- Source: https://www.thinkwithprompt.com/prompts/read-my-spc-chart-and-process-capability-results

## Prompt

```text
Help me interpret statistical process control (SPC) data and capability results.

**Characteristic and specification:** [e.g. "bore diameter, 25.00 mm +/- 0.05"]
**Data:** [PASTE measurements in time order with subgroup IDs, or the chart's summary values]
**Chart type used:** [X-bar and R / X-bar and S / I-MR / p or np chart / not sure]
**Subgroup size and sampling frequency:** [e.g. "5 parts every hour"]
**Capability results if already calculated:** [Cp, Cpk, Pp, Ppk]
**Customer requirement:** [e.g. "Cpk 1.33 minimum, 1.67 for special characteristics"]
**Process notes:** [tool changes, material lots, shifts, adjustments during the period]

Steps:
1. Check the chart type fits the data (variable vs. attribute, subgroup size) and say if a different one is better.
2. If data is given, calculate the center line and control limits, and show the formulas.
3. Check for out-of-control signals using the Western Electric/Nelson rules (points beyond limits, runs of 7-9 on one side, trends, cycles, hugging the center line). Link signals to my process notes where possible.
4. Only if the process is stable, interpret capability: Cp vs. Cpk (spread vs. centering), short-term vs. long-term (Cpk vs. Ppk), expected parts out of spec. Warn if data looks non-normal or the sample is too small (under about 100 values) to trust the result.
5. Recommend actions: center the process, reduce variation, investigate special causes, or reconsider the tolerance with engineering.

Return: a verdict (stable? capable?), the signals found, capability interpretation, next actions, and a 3-sentence summary for a non-statistician.

Do not invent data points. Remind me that control limits come from the process, not the specification.
```

## What to fill in

- e.g. "bore diameter, 25.00 mm +/- 0.05"
- X-bar and R / X-bar and S / I-MR / p or np chart / not sure
- e.g. "5 parts every hour"
- Cp, Cpk, Pp, Ppk
- e.g. "Cpk 1.33 minimum, 1.67 for special characteristics"

Free to copy and use. From ThinkWithPrompt (https://www.thinkwithprompt.com/), a library of 2,663 curated AI prompts.