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What is Cp/Cpk?

Monitoring the efficiency and performances of our manufacturing line is very important. It helps us find the unstable processes and improve them to increase our yield.
One important way to measure the performances is using Cp/Cpk.

Cp/Cpk are 2 statistical parameters that help us get a very good indicator of the stability and capability of our process.

In short, the main 2 ideas here is measuring the variation of our processes (the smaller the better) and how centered the processes are (How close the mean is to the center line between the upper and lower limits defined for each process).

Cp stands for Process Capability. A direct way to measure process capability. It measures how close your process is to the defined upper and lower limits of the process, compared to a reference optimal process. The larger the Cp index is , the less likely it is that any unit manufactured in your process will be outside the defined limits.

Cpk stands for Process Capability Index. It is the same as Cp  but adjusted  to measure process which is analysed with a non-centered distribution.  Cpk measures how close you are to the optimal area of your process and how consistent your process is. The larger the Cpk index of your process  is , the less likely it is that any unit manufactured in your process will be outside the defined limits.

How Cp, Cpk   related to each other?

Your process perform with minimum variation, but it can be away from the optimal target towards one of the defined High or Low limit. This indicates a high index of Cp whereas Cpk will be low. On the other hand, your process  may be on average exactly at the optimal target, but the variation level is high . In this scenario Cpk will also be lower, but Cp will be high. Cpk will be higher only when your process meets the optimal target consistently with minimum variation.

Here is a  “real life” example of using Cp/Cpk of a manufacturing line of digital cameras:

  • The operating temperature of the camera should be between -10°C and 45°C when it is delivered to customers.
  • The testing process that is been used to measure the operating temperature during the manufacturing process process has a process standard deviation of 3°C.
  • The average temperature (mean value) is 28°C .
  • High limit (Upper Specification Limit) is 45°C.
  • Low limit (Lower Specification Limit) = -10°C

What is the capability of the process?

In order to get the answer, we just need to put the numbers in the above simulator and view the results.
As you can see in the chart, the distribution is not centralized and therefore manufacturing process of our digital camera is not stable enough and may cause quality issues. It means that we need to either change the something in the camera or in process to better centralized it (different components, change the technical design, etc.)