Capability: Cp, Cpk and process variation
Compare process variation with specification requirements.
What problem it solves
A Cpk value alone is not enough to declare a process capable. Process stability, measurement validity and the distribution model must be assessed before interpretation.
What we can configure in QAPP
- Data collection with explicit units and specification limits.
- Histogram, distribution and capability indicators.
- Interpretation together with sampling context and process history.
The module is part of QAPP, not a separate product. Included features, rules, reports and integrations are confirmed in the project proposal and validated at acceptance. This presentation does not certify automatic compliance with any standard.
How to start
Send us a form without confidential data and describe who completes it, how often, and which decisions are based on it. Together, we will define a relevant example for the demonstration.
Methodological reference: NIST/SEMATECH e-Handbook.
Request a demonstration →Practical guide: when to use it
Check whether the variation of a stable process fits within the required limits.
Data and preparation
Numerical measurements, unit, chronological order, LSL and USL limits, and the sampling method. Limits come from requirements, not from the minimum and maximum values in the data.
How to work, step by step
Cp = (USL − LSL) / (6σ within). Cpk = min((USL − μ)/(3σ within), (μ − LSL)/(3σ within)). Pp and Ppk use overall variation. The sigma estimator must be specified; the results are not interchangeable.
- Define the process and the decision the tool needs to support.
- Check data sources and data quality before analysis.
- Apply the method and document assumptions and responsibilities.
- Validate the result with the team and define the next action.
Worked example
With LSL 90, USL 110, μ 102 and σ within 2, Cp is 1.67 and Cpk is 1.33. The difference shows that the mean is shifted towards the upper limit. This is a hypothetical example, not a result calculated from 15 data points.
How to interpret it correctly
Do not interpret Cpk as proof of stability. First check the control chart, measurement system and distribution. The acceptance threshold is the one agreed for the process; a generic value does not replace the customer requirement.
What QAPP can connect
The form, records and evidence can be connected to an owner, deadline and report. Fields and automations are defined for your process; examples on the website are illustrative.
What should I prepare for a demonstration?
An anonymised example, the current rules, who enters the data and which result you want to track. We can define a pilot with verification criteria before expanding.
Can I use the demo to validate the process?
The demo helps explain the method. Validating a real process requires representative data, verified assumptions and agreed criteria.