PPK vs CPK: Difference and Comparison

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.

Key Takeaways

  1. Ppk and Cpk are statistical process capability indices used to measure a process’s performance.
  2. Ppk measures the capability of a process to meet customer specifications, while Cpk measures the capability of a process to stay within the specified control limits.
  3. Ppk and Cpk are used in quality control to improve processes and reduce defects.

PPK vs CPK

PPK is an index that is used to measure exactly how a process is running and to calculate the process variations. You use the actual standard deviation approach. CPK is an index that is used to measure what a process can or cannot do and you use an estimated standard deviation to calculate it.

PPK vs CPK

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.

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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

Parameters of ComparisonPPKCPK
MeaningPPK 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.
CalculationIt uses actual standard derivation (actual sigma) to calculate process variations.It uses estimated standard deviation (estimated sigma) to calculate process variations.
InterpretationIt interprets short-term capability.It interprets long-term capability.
ControlThe 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.
ConsiderationThe 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 by 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 quickly and economically. The method is a useful metric to evaluate the progression of a set-up.

However, there is a risk factor involved, as the index infers that the process is ready for production before all of its intricacies have been worked out.

It is always resourceful to use the PPK and PP indexes. Both 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 standard deviation estimation is calculated by dividing the average range with a tabular constant.

Near or below zero values show that the process is off-target and 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 determine whether their processes will work well. 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 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 indexes depicts that the process is not centred within specification limits.

Main Differences Between PPK and CPK

  1. PPK refers to an index that measures how a process is running, whereas CPK measures the potential of the process.
  2. PPK uses actual standard deviation to calculate process variations, whereas CPK uses estimated standard deviation.
  3. PPK interprets short-term capability, whereas CPK interprets long-term capability.
  4. 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.
  5. PPK ignores time as a factor, whereas CPK considers time in its calculation.
References
  1. https://www.spiedigitallibrary.org/conference-proceedings-of-spie/4889/0000/Cpk-and-Ppk–one-capability-index-is-not-enough/10.1117/12.467579.short
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