Process Capability Analysis
Process Capability assesses whether a stable process can meet specification limits — quantifying how well natural process variation fits within the tolerance band.
Step 1 — Provide data and specifications
- Process data — measured values
- USL — upper specification limit
- LSL — lower specification limit
- Target (optional) — ideal value
- Subgroup size (default 1)
Supports Excel Import, Sample Data Generator, and Manual Entry.
Step 2 — Results
Capability indices (within-subgroup variation):
- Cp — potential capability (assumes centred)
- Cpu / Cpl — one-sided
- Cpk — actual capability (accounts for centring)
Performance indices (overall variation):
- Pp, Ppu, Ppl, Ppk
Defect estimates:
- PPM above USL, below LSL, total
- Percentage out of specification
Visualisations: Histogram with normal curve and spec limits.
A UniversalChatBot is available for discussion.
Statistical methods used
Capability indices:
Cp = (USL - LSL) / (6 * withinSD)Cpk = min((USL - mean)/(3*withinSD), (mean - LSL)/(3*withinSD))
Within SD: From mean moving range (MR-bar / 1.128) or mean subgroup range (R-bar / d2).
Interpretation:
| Cpk | Label |
|---|---|
| >= 2.00 | Excellent (Six Sigma) |
| 1.67 - 2.00 | Very Good |
| 1.33 - 1.67 | Good |
| 1.00 - 1.33 | Marginal |
| < 1.00 | Poor (not capable) |
Defect rate: Based on normal distribution z-scores at spec limits.
Important: Capability indices only meaningful if process is in statistical control first.