Ppk stands for Process Performance Index and is used for long-term variability.
Overview: What is Ppk?
Ppk is a performance index that is used to see if a current process is delivering acceptable results. It does this by measuring how closely the specification limits are in proximity to the mean of the current process.
Ppk is calculated in one of two ways:
If your central tendency is closer to the Upper Specification Limit (USL), you will find the Ppk with this equation:
Ppk = [ USL – x(bar) ] / 3 s
In this equation, ‘x(bar)’ is going to be your central tendency or Process Mean.
If your central tendency is closer to the Lower Specification Limit (LSL), you will determine the Ppk this way:
Ppk = [x(bar) – LSL ] / 3 s
‘x(bar)’ is going to be your central tendency or Process Mean in this equation also.
3 benefits of Ppk
There are some major benefits to using indices like Ppk:
1. Less scrap
Indices like Ppk help you to understand process behavior, which helps you to produce less scrap
2. Cost reduction
By using Ppk, you can help reduce production costs and loss due to quality being less than ideal.
Using Ppk can help you make future plans based on how well your process is meeting requirements over a period of time.
Why is Ppk important to understand?
Ppk is an important index to understand for the following reasons:
It is a valuable performance indicator
Having a solid understanding of Ppk is important because it is a good indicator of how a process performs over a period of time.
Understanding Ppk values can tell you if a process is meeting customer requirements
If the Ppk is greater than 1.67, the process is probably meeting customer requirements. If the Ppk is less than or equal to 1.67, then the process may not meet the requirements of the customer. If the Ppk is less than 1.33, then the process should be deemed substandard and process improvements should be implemented with as high a priority as possible.
Having a working knowledge of Ppk and other indices will help you interpret the data that the indices are relaying to you. For example, if the Cpk and Ppk values vary greatly, then it is likely that your process is not under statistical control.
An industry example of Ppk
A company produces axles with a diameter of 20 mm. With the understanding that the machines being used will not make the axles exactly 20 mm most of the time, it is specified what the maximum admissible deviations or specification limits are. The Ppk measures how the produced axle satisfies requirements over a period of time.
3 best practices when thinking about Ppk
Here are some practices to keep in mind when it comes to Ppk:
1. Ppk in relation to Z Scores
You can find the Ppk by dividing a Z Score by 3.
2. Ppk is not the best estimator of the potential for your process
For this, you are going to want to use Cpk as it is a representation of the best your process is likely to do.
3. Remember that Ppk is not measured in single standard deviation units
Instead of being measured in single standard deviation units, of which it is generally accepted that you want to have at least six between the mean and the nearest spec limit, Ppk is measured in three sigma sets.
Frequently Asked Questions (FAQ) about Ppk
When was the concept of process capability first introduced?
The concept of process capability was first introduced in 1956 in the ‘Statistical Quality Control Handbook’.
Why is Ppk higher than Cpk?
Ppk, with few exceptions, is higher than Cpk because it is a long-term capability while Cpk is for the short-term.
What does a high Ppk mean?
Generally speaking, having a high Ppk value means having a more capable process. With a lower Ppk value, it could be an indicator of a need for improvement in the process.
Ppk as a tool for your business
Knowing that your process is satisfactorily meeting requirements is a powerful tool for the health of your business. Ppk is a valuable index that helps you make that determination.