There are anomalies seen when the sample size is small and the data represents a short amount of time where estimating using R will overstate standard deviation and make Cpk smaller than Ppk. In theory Cpk will always be greater than or equal to Ppk. Process Performance generally uses sample sigma in its calculation. Because there is not a lot of historical data we take large samples from the process to account for variation. there is a short pre-production run or you are piloting a new process. Generally you use this when a process is too new to determine if it is under statistical control. Pp and PPk are used for Process Performance. Process capability uses the process sigma value determined from either the Moving Range, Range or Sigma control charts This often happens with a mature process that has been around for a while. Generally you use this when a process is under statistical control. In both cases we want to try to verify if the process can meet to meet Customer CTQs (requirements).Ĭp, and Cpk are used for Process Capability. Pp and Ppk are called Process Performance. What is the Difference between Cp, Cpk and Pp, PPk?Ĭp and Cpk are called Process Capability. Again, this will help me make the article better for you and everyone else. If you’re studying for something else, please leave a comment or contact me and let me know what you are studying for. This will help me make the article even better for you. If that’s you, leave me a comment below or contact me and let me know which organization and belt you’re studying for. This article was written to help Six Sigma Green Belt and Black Belt candidates prepare for and pass their exams. If you are not finding what you are looking for, please let me know in the notes below. There are also crib notes on what the equations mean in a real performance sense, what you should be able to tell about a process depending on Cp and Cpk values and more. This is a long article, but I thought it was important to keep Cp and Cpk together. In other words, it allows us to compare apple processes to orange processes! Process Capability
In Six Sigma we want to describe processes quality in terms of sigma because this gives us an easy way to talk about how capable different processes are using a common mathematical framework.
Cp and Cpk are considered short-term potential capability measures for a process.
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