PPK and CPK are two different indexes that are involved in process capability analysis. Several processes carried out in an organization have inherent statistical variability. The indexes are parts of the methods that are used to evaluate them. However, telling them apart can often be confusing especially for those who are not regularly working with them.
PPK vs CPK
The main difference between PPK and CPK is that PPK is short for Process Performance Index which evaluates how a process has performed and whether it is meeting specifications or not. On the other hand, CPK is short for Capability Performance Index which predicts whether a process has the potential to meet a certain specification.
PPK, essentially, is an index that measures how well a process has performed over a period of time. Its calculations involve actual sigma which shows the running of the system with respect to the targeted specifications. A greater PPK means that the difference between process output and specification limit is less.
CPK measures the potential of a process and tells us how well it can meet the specifications. Its calculations involve estimated sigma which helps predict whether the process will meet specification limits or not. Like the former, a greater CPK means that the difference between process output and specification limit is less.
Comparison Table Between PPK and CPK
| Parameters of Comparison | PPK | CPK |
| Meaning | PPK refers to an index that measures how a process is running. | CPK refers to an index that measures what a process is capable of doing. |
| Calculation | It uses actual standard derivation (actual sigma) to calculate process variations. | It uses estimated standard deviation (estimated sigma) to calculate process variations. |
| Interpretation | It interprets shot-term capability. | It interprets long-term capability. |
| Control | The process is not in a state of control and thus cannot predict the future. | The process is in a state of control and can thus predict the future. |
| Consideration | The index does not consider time as a factor. | The index considers time as a factor. |
What is PPK?
PPK is a statistical tool that helps evaluate process capability in the phase of initial set-up. It is brought into effect before the process is in a state of control. Thus, it cannot predict the future. Rather, how a process is actually running and whether it is meeting the specification limit. Essentially, it evaluates the short-term capability of a process.
The calculations in this method are carried out using actual sigma. This is a standard deviation that is calculated from using individual data values present in a data set. If the value of PPK is large, this means that the process is more capable of producing the required output. Meanwhile, if the value of PPK is less, the process is not as capable of producing output.
PPK is most commonly used when an organization needs to set up an entirely new process in a fast and economical manner. The method is a useful metric to evaluate the progression of a set-up. However, there is a factor of risk involved as the index infers that the process is ready for production way before all of its intricacies have been worked out.
It is always resourceful to use the PPK index along with the PP index. Both the metrics must be used in conjunction when the process is centred on its target value. Their equivalency depicts that the process is centred within specification limits.
What is CPK?
CPK is another statistical tool that evaluates the potential of a process. It measures whether the process will be able to produce output when it is in a state of control. Natural variances in the system are calculated with respect to specification limits. This shows how well an organization will be able to control the process.
The calculation of CPK is carried out using estimated sigma. This is an estimation of standard deviation that is calculated by dividing the average range with a tabular constant. Values that are near or below zero show that the process is off-target and that the variation is high. Meanwhile, values that are greater than zero show that the process is on target with less variation.
CPK index is commonly used when organizations need to figure out whether their processes will work well or not. There is no haste or urgency involved. However, the evaluation is only meaningful when the variations are natural. If other unexpected variations or errors in the process occur, the process capability will have no real value.
It is always resourceful to use CPK along with the CP index. Both the metrics must be used in conjunction when the process is centred on the target value. Unlike in the case of PPK, an equivalency in both the indexes depicts that the process is not centred within specification limits.
Main Differences Between PPK and CPK
- PPK refers to an index that measures how a process is running whereas CPK measures the potential of the process.
- PPK uses actual standard deviation to calculate process variations whereas CPK uses estimated standard deviation.
- PPK interprets short term capability whereas CPK interprets long term capability.
- PPK is meaningful when the process is not in a state of control whereas CPK is meaningful when the process is in a state of control.
- PPK ignores time as a factor whereas CPK considers time in its calculation.
Conclusion
PPK and CPK are two indexes used in statistical calculation. Understanding them can be difficult for those who are not accustomed to using them on a regular basis. However, the simplest difference between them is that PPK measures how a process is running whereas CPK measured the potential of the process.
Another difference between the two is that they use different methods of calculation. While PPK uses actual standard deviation, CPK uses estimated standard deviation. The inferences of the values differ as well. PPK can only interpret short-term capability whereas CPK can interpret long-term capability of the process. Moreover, PPK ignores time as a factor while CPK does not.
References
- https://www.spiedigitallibrary.org/conference-proceedings-of-spie/4889/0000/Cpk-and-Ppk–one-capability-index-is-not-enough/10.1117/12.467579.short