Cp (Process Capability Index)
Ratio of specification width to short-term variation — ignores the location of the mean.
Also known as: Cp value, potential capability
Cp measures the potential capability of a process: does its natural short-term variation even fit inside the customer's limits? It says nothing about centring — that is Cpk's job.
- Cp = 1 — process spread exactly covers the tolerance (0.27 % outside under perfect centring)
- Cp = 1.33 — Automotive minimum
- Cp = 1.67 — Six Sigma target
- σ_within is estimated from subgroups (Rbar/d₂ or Sbar/c₄), not from the total sample
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