Track 15 · Applied Statistics for Professionals · Lesson 8 of 12
Process capability
Calculate Cp, Cpk, Pp and Ppk, translate them into expected nonconforming parts, and judge when a capability index can be trusted.
A customer asks the workshop: "Can your lathes hold 25.00 ± 0.05 mm?" A control chart says whether a process is stable; a capability index says whether that stable process fits inside the customer's tolerance — and by how much margin. Suppliers are often required to report Cpk ≥ 1.33, so the number carries contractual weight.
Capability compares two widths: the tolerance the customer allows (USL − LSL) and the natural spread of the process, conventionally (about 99.73% of a normal process lies within ). If the natural spread fits comfortably inside the tolerance, and the process is centred, almost every part conforms.
Included with a Formula Quest membership
It has 8 graded exercises with feedback, hints and worked solutions. After it you will be able to:
- Calculate Cp, Cpk, Pp and Ppk and explain the difference between within and overall σ
- Translate a capability index into an approximate nonconforming rate under normality
- Diagnose whether poor capability comes from centring or from spread
- Refuse to interpret capability for an unstable, non-normal or poorly measured process