Cpk (Centred Process Capability Index)
Extends Cp by the location — assesses the actual short-term capability accounting for off-centring.
Also known as: Cpk value, centred capability
Cpk is the standard short-term capability metric. It takes the smaller of the two distances {mean to upper spec} and {mean to lower spec}, normalised by 3σ.
- Cpk < 1 — not capable
- Cpk 1.00–1.33 — marginal
- Cpk 1.33–1.67 — good
- Cpk > 1.67 — very good (Automotive target)
- Cp = Cpk ⇔ process exactly centred
See also
Used in
In the Algorithm Lab
Sources
- Montgomery, D. C. — Introduction to Statistical Quality Control, 8th Edition, Chapter 8
- AIAG — Statistical Process Control (SPC) Reference Manual, 2nd Edition
- Kane, V. E. — Process Capability Indices, Journal of Quality Technology, 18(1), 1986