[Potential] Process Capability Analysis (Cp, Cpk)
A process capability study uses data from a sample to PREDICT the ability of a manufacturing process to produce parts conforming to specifications.
This prediction enables us to “qualify” a new manufacturing process as being fit for use in production. Therefore, Process Capability Analysis is best used during New Product Introduction (NPI), or at the start of each day or each shift of production as a method of checking whether a process is in control and capable of producing parts that meet specifications.
[Actual] Process Performance Analysis (Pp, Ppk)
A process performance study is used to EVALUATE a manufacturing process and answers the question: “how did the process actually perform over a period of time?” In daily use we tend to omit an important word, "actual", from the name of this analysis.
The missing word “Actual” from Process Performance and the missing word "Potential" from Process Capability result in significant confusion among practitioners.
The Process Performance Analysis is best used when you have inspection data, and you want to use the inspection data to understand how your manufacturing process actually performed. This is a historical analysis rather than a predictive analysis, but can still be used to drive process improvements.
For additional information, please see the following blog posts: