Capability Studies and Cpk Improvements are a Game Changer!
Understanding six-sigma and process capability is essential for factory leaders at all levels. Processes variation is a fact of life in a plant. Measuring and understanding process variation is the first step in process improvement.
The three main measurements used in a process capability study are Cp. Cpk and Ppk. Each of these measures are related to standard deviation so if you understand how process variation is measured using the standard deviation then you can easily understand the three main measurements used in process capability studies.
If a process is centered and there are no shifts or drifts in the process due to assignable causes, then there is an easy connection between standard deviation and each of the three process capability measures.
When the process is centered to the specs and the process isn’t shifting all over the place, then Cp, Cpk and Ppk will all be very close numbers and each standard deviation represents 0.33 Cp, Cpk and Ppk.
At a high-level each standard deviation represents 0.33 in your process capability model. So a process that is running at 3 standard deviations has a process capability of 1.0 (cp, cpk and ppk) and a defect rate of 67,000 per million units produced. A process that is running at six standard deviations or six-sigma has a process capability of 2.0 (cp, cpk and ppk) and a defect rate of only 3 defects per million units produced.
What is the difference between Cp, Cpk and Ppk?
Cp and Cpk are used for short term studies. These measures are used to establish your very best-case process (aka your benchmark or entitlement).
To perform a short-term process capability study, you want to make the machine as perfect as possible via a PM and use raw materials that are known to be good and you want you most qualified operator running the machine. Then you will produce at least 30 samples in a row and then measure those samples using a qualified inspector.
Cp and Cpk will establish your benchmark. Your baseline Cp and Cpk will be used in the future to compare your long-term process capability results.
The difference between Cp and Cpk is that Cp assumes that the data is centered but Cpk is calculated by not assuming the process can be centered. So if your process is not centered Cpk will be lower number than Cp.
What is Ppk? Ppk is used for long term studies. For a long-term study, we need long-term data that is gathered over a long period of time. The larger the sample size and data set, the better.
What is the difference between the calculation of Cpk and Ppk? The difference is that Cpk using a rolling format to calculate the standard deviation and Ppk uses the typical method to calculate the standard deviation.
It’s not easy to understand, but just know that if your process has a lot of shifts and drifts due to assignable causes like operator differences and raw material changes, Cpk won’t reflect these process changes, but Ppk will reflect them. The idea is that a process engineer can compare the Cpk to the Ppk and know if they have process improvement opportunities.
Process capability is simple really. Just remember these few things:
- Cp is a short-term measure and it represents your absolute best process. It assumes perfect process conditions and that your process is centered.
- Cpk is a short term measure and it represents your optimal process and it becomes your long-term goal. Cpk does not assume centering so always compare Cp to Cpk to see if you have a centering issue with your process. Most companies target a minimum Cpk of 1.33.
- Ppk is used in a long-term study. Compare Ppk to Pp to see if your process is centered and then compare your long-term Ppk results to your short term Cpk results and see if the results are close to each other. If not, then your process has changed over time. Maybe your machine is wearing down or operators need training or your raw materials are inconsistent.
- When your Ppk doesn’t match your initial Cpk results or a 1.33 Ppk then this situation provides an opportunity for a team-based process improvement project. The question for the team is; "Why isn’t the process running as well now as it was originally - when we ran the short term process capability study?" This is what a Six-Sigma project is all about - reducing process variation!
Lean and six-sigma offer a very large toolbox for factory leaders. Concepts in Manufacturing Leadership is a series of 4 Certification Courses that are proven to help Supervisors build a solid and successful career in Manufacturing Leadership. Get certified in Concepts in Manufacturing Leadership!
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