Capability Analysis

Process Capability- Cp, Cpk

Capability can be assessed for either variable or attribute data collected either in the short-term or in the long-term. The process must be in control before you assess capability. If the process is not in control, then, the capability estimates will be incorrect.

  • Assess capability for a quantitative/qualitative parameter

1. Verify the specification

The spec is ..., The tolerance is ..., Is it okay to ask the customer? 

2. Collect a data sample

long-term or short-term?

qualitative parameter)
Count the occurrence of defects
Caluculate the proportion of defects
Caluculate Z-score

3. Calculate Cp, Cpk

qualitative parameter) change from Z to Cp, Cpk, etc


  • Estimating Cp, Cpk

  • The short-term or long-term? -> Z-Shift
Z-Shift-> ZLT=ZST-1.5

*ST:short-term, LT:long-term
If the initial data was given as 3s in the short- term, the conversion to long- term would be as follows: 3s-1.5s=1.5s
Other conversion:Cpk=ZST/3, Ppk=ZLT/3, ZST=1.5+ZLT


Process capability index (Wikipedia)
6.1.6 What is Process Capability?
NIST/SEMATECH e-Handbook of Statistical Methods,
● Z Shift
工程能力分析-Capability Analysis (Japanense)