Process Capability & Performance Indices Cp, Cpk , Pp and Ppk

Cp (Process Capability)

Meaning:

• Measures potential capability

• Assumes process is centered

Interpretation:

• Cp = 1 → Just meets specs

• Cp > 1 → Good capability

• Cp < 1 → Not capable

👉 Limitation: Ignores process shift

Cpk (Process Capability Index)

Meaning:

• Measures actual capability

• Considers process centering

Interpretation:

• Cpk = Cp → Process perfectly centered

• Cpk < Cp → Process shifted

• Cpk < 1 → Defects likely

Pp (Process Performance)

Meaning:

• Similar to Cp, but uses overall variation (s)

• Includes long-term variation

Ppk (Process Performance Index)

Meaning:

• Real-world performance including:

  • shifts

  • drifts

  • instability

Cp/Cpk vs Pp/Ppk

Cp --> Within variation (σ) --> Short-term capability

Cpk--> Within + centering --> Short-term actual capability

Pp --> Overall variation (s)--> Long-term performance

Ppk--> Overall + centering --> Real-world performance

Summary:

• Cp-> Potential Capability (Precision of the Process)

• Cpk-> Actual Capability (Accuracy of the Process)

• Pp/Ppk-> Long term process behaviour

Industry Benchmark:

• Cpk/Ppk < 1.0 --> Not Capable

• Cpk/Ppk = 1.0 --> Bare minimum

• Cpk/Ppk >=1.33 --> Acceptable

• Cpk/Ppk>= 1.67 --> Good

• Cpk/Ppk >=2.0 -> Excellent

Practical Example:

• Cp = 1.5 but Cpk = 0.8 --> Process has potential but is off-center

• Cpk = 1.4 but Ppk = 0.9 --> Process unstable over time

• Cp=2.0 and Cpk=0.9 --> Process capable but badly centred

• Cpk=1.5 and Ppk=1.0 --> Process unstable over time

• Cpk=1.33 --> Good and acceptable

• Ppk<1--> Customer likely seeing defects

Sample Graphs