Process Capability Analysis tells us how well a process meets a set of specification limits, based on a sample of data taken from a process. What Can Process Capability and Process Performance Tell Us? In Cp we are assuming a stable process and will likely have enough data to calculate a true standard deviation. Pp we use sampling and have to calculate an estimated standard deviation of the sample. The biggest difference between Cp and Pp is how the Standard Deviation is determined. When to use Pp Ppk Cp Cpk Differences Between Cp & Pp

To determine the answer to those questions we see how wide the process dispersion (spread of values) and how centered the process is relative to those specifications. To begin to answer these questions it all starts with customer requirements and the follow up with what we know mathematically about the variation of the process.Ĭp, Cpk, Pp and Ppk are all parameters (indices) that can help us to understand how our process is operating relative to the specifications, or in other words, they measure how close our process is running to its specification limits.įor requirements we measure the process specifications. If you’d like more depth including calculations, etc, see these articles:

This article deals with an overall examination of Process Capability & Performance (Pp, Ppk, Cp, Cpk). Is a process capable? Is the process acceptable? How is the process actually behaving vs how it theoretically could be behaving? These are questions you can ask during a DMAIC project during both the measure phase (if you have good, existing data) or in the control phase after you have implemented your changes. That's why it's more common for aerospace and sometimes automotive companies to push 5 and 6 Sigma levels but only in cases where the cost of product failure justifies that level of process control.Cp, Pp, Cpk, and Ppk are capability and performance indicators for your process. In-fact the cost of making improvements beyond a 4 Sigma level may not be worth the investment. Many companies might be willing to run with scrap rates around 0.62%. Notice the margin of diminishing return between 3 and 4 Sigma. Many practitioners will be more interested in long-term defect rates as these are commonly used in cost-benefit analysis by management making critical business decisions invovling how to invest capital and so forth. Notice the difference between short-term and long-term is an industry standard 1.5 shift of the Sigma Level. Two tables below provide both short-term and long-term conversions of Sigma Level (Z-Score), CPK or PPK, and Defect Rate respectively.

Cpk vs ppk 1.33