Process capability answers: given the natural variation of my process, how consistently will it produce output within specification?
Cp - the potential
Cp measures the spread of your process against the width of the spec. Cp of 1.0 means the process spread exactly fills the spec (marginal). Cp of 1.33 is a common target (comfortable). Cp of 2.0 is Six Sigma territory.
Cpk - the actual
Cpk is Cp adjusted for centring. A process can be tight (high Cp) but off-centre (low Cpk). Cpk is always less than or equal to Cp. If Cp is 2.0 but Cpk is 0.5, the process is precise but consistently missing the target.
What to do with them
- Cp < 1: reduce variation. The process is fundamentally too spread out.
- Cp >= 1 but Cpk < Cp: recentre. The variation is fine; the aim is off.
- Both are high: leave it alone. Better places to spend improvement effort.