Let’s take the confusion out of Cpk and Ppk.
Just like control charts tell us about the stability of the process, capability analyses tell us about process capability. Capability Indices help us judge whether the process is capable of producing parts that are within specifications.
Among the various capability indices, Cpk and Ppk are popular in the manufacturing world. However, many users report they are also the most confusing.
WinSPC Means Lower Costs and Higher Quality
WinSPC is software to help manufacturers create the highest quality product for the lowest possible cost. You can learn more here.
These two indices consider process centering to calculate the process capability.
The formula for the Cpk and Ppk are:
So which capability index should you use, Cpk or Ppk? Although they show similar information, they have slightly different uses.
Cpk uses an “estimated sigma” value in its formula. It is commonly used to measure whether the process is capable of meeting a customer’s requirements.
Ppk uses the “actual sigma” value in its formula so it is commonly used to measure whether the process is actually meeting a customer’s requirements.
Six Sigma Blackbelt
Download the white paper on how to jump-start a “mini” Six Sigma Quality program on a budget