The CPK (Process Capability Index) is a statistical indicator used to measure how well a process is performing in relation to its tolerance specifications. It shows whether the process is centered and stable enough to produce within the acceptable limits defined by the design or standard.
Why is CPK important?
In industrial environments, test laboratories, and production lines, CPK allows for evaluating the actual quality of the process, identifying excessive variations or trends outside the specification center. It is essential for:
- Preventing production failures
- Improving the quality of the final product
- Validating new processes (as in NPI)
- Supporting quality audits (ISO, IATF, FDA…)
- Making data-driven decisions for continuous improvements
📐 Cpk Formula
🧩 Where:
- USL = Upper Specification Limit
- LSL = Lower Specification Limit
- μ (mu) = Process mean
- σ (sigma) = Process standard deviation
- Cpk = Capability index considering process centering
How to interpret the CPK value?
| CPK Value | Interpretation |
|---|---|
| ≥ 2.00 | Excellent process – fully under control |
| ≥ 1.67 | Very good process – accepted in demanding sectors |
| ≥ 1.33 | Acceptable process – meets quality standards |
| < 1.33 | Unstable process – requires investigation/improvement |
| < 1.00 | Out of control process – risk of non-compliance |
Application of CPK with AJOLLY Testing
At AJOLLY Testing, we use CPK calculation as part of the test data analysis produced by test software to ensure the stability of measurement systems, jigs, and benches in production and validation.
Examples of practical application:
- Measurement of output voltage of a regulated power supply in functional tests
- Monitoring of response time of sensors or actuators
- Control of current consumption of an electronic device over batches
- Verification of repeatability of results between stations and operators
Benefits of CPK analysis
- Early identification of process drifts
- Reduction of waste and rework
- Statistical basis for continuous improvement (PDCA, Six Sigma)
- Support for traceability and product validation
- Better decision-making in production management
🔁 CPK vs PPK: what’s the difference?
Although CPK (Process Capability Index) and PPK (Process Performance Index) are often used together, they serve different purposes:
- PPK, on the other hand, evaluates the actual performance of the process based on current data, even if the process is not fully stable.
- CPK calculates the theoretical capability of a process that is under statistical control — meaning it is stable and predictable.
In practice, AJOLLY Testing uses both to ensure:
- The process has the potential to meet tolerance limits (CPK)
- And that it is actually performing according to specifications (PPK)
If CPK is high but PPK is low, it means the process has capacity, but something is currently drifting, such as an operator change, jig instability, or a firmware issue.