March 15th, 2019
Author: Kelly Jakinovich - Medical Extrusion Product Manager
Understanding these concepts can be tough when you aren’t regularly working with them. This article is not how to calculate Cpk or Ppk, as there are several resources in print or on the internet to figure that out. Instead this article is intended to explain the difference between the two and why the different calculations are important.
In the short sense, Cpk and Ppk are about process capability and process performance, respectively.
Process capability, Cpk, is important because it indicates whether a process potentially can meet a specification. Essentially, it is a prediction of the ability of a process to meet a specification.
Process performance, Ppk, is important because it indicates how the actual process performed over a period of time. Essentially, it is an evaluation of how well a process performed.
So the key takeaway is that Cpk is the potential of a process to meet a specification (short term) while Ppk is how the process actually did (long term).
Another way to look at the difference is that Cpk is used for a subgroup of data, while Ppk is used for the whole process. Cpk is typically used while processing in the ideal conditions to identify if the process is capable of meeting the specifications. Ppk tends to be more realistic because it doesn’t cut out real data, while Cpk data gathering could inadvertently skew the data into looking like a process is capable but not addressing its performance.
When the Cpk calculation is a high number (general industry guideline is 1.66 or higher), then it is a good indication the process is statistically in control. If the Cpk and Ppk calculations are very close or equal, then there is an even better indication that the process is statically in control. However, if the two calculations are considerably different, even though the Cpk value may be acceptable, this is an indication that the process is not in statistical control because there are more data points that have been evaluated in actual process conditions. This is a very important piece of information! Long term, it’s not clear that consistent parts are being produced. Even though they may be within the specification limits, the parts could be at the low or high end of the specification. If a process cannot handle that variation well, this could cause all sorts of issues such as installation challenges, performance issues, and other quality issues, which all cost a lot of time and money.
One thing to note is that many people and even resources will confuse the definitions. You will always want to make sure that the potential versus actual is defined so the same language is spoken between the parties. This will ensure that everyone is on the same page.