Q 609. Why is that we need to look at 2 capability stats - both Pp and Ppk (or both Cp and Cpk)? What are the challenges if we look at any one of them alone? Support your answers with examples Note for website visitors - This platform hosts two weekly questions, one on Tuesday and the other on Friday. All previous questions can be found here: https://www.benchmarksixsigma.com/forum/lean-six-sigma-business-excellence-questions/. To participate in the current question, please visit the forum homepage at https://www.benchmarksixsigma.com/forum/. The question will be open until Tuesday or Friday at 5 PM Indian Standard Time, depending on the launch day. Responses will not be visible until they are reviewed, and only non-plagiarised answers with less than 5-10% plagiarism will be approved. If you are unsure about plagiarism, please check your answer using a plagiarism checker tool such as https://smallseotools.com/plagiarism-checker/ before submitting. All correct answers shall be published, and the top-rated answer will be displayed first. The author will receive an honorable mention in our Business Excellence dictionary at https://www.benchmarksixsigma.com/forum/business-excellence-dictionary-glossary/ along with the related term. Some people seem to be using AI platforms to find forum answers. This is a risky approach as AI responses are error prone as our questions are application-oriented (they are never straightforward). Have a look at this funny example - https://www.benchmarksixsigma.com/forum/topic/39458-using-ai-to-respond-to-forum-questions/ We also use an AI content detector at https://crossplag.com/ai-content-detector/. Only answers with less than 15-20% AI-generated content will be approved.

This question has thrown up surprises. It seemed easy but still there are a few answers that have not been approved because they were incorrect. There are also a couple of answers that were not approved due to AI generated content. Anyway, lets come to this question, the remaining answer from Adil Khan has been selected the best answer. However, want to add a few points 1. Looking alone at Pp (or Cp) is detrimental for two reasons It does not consider the process centricity It cannot be calculated for single side specifications 2. Some experienced practitioners only look at Ppk (or Cpk) value. since Ppk value will always be less than equal to Pp, therefore they work with a criteria of Ppk to be greater or equal to 1.33 (the number 1.33 could be different for some industries). This will ensure that Pp value is also acceptable. Keep in mind that process might still be off centered but is still capable of meeting customer specifications.

Message added by Mayank Gupta, October 20, 20232 yr

Capability Stats (Pp/Ppk or Cp/Cpk) are metrics that can be used to quantify the capability of processes. To obtain these indices the process performance is compared with the customer requirements.

Performance Indices (Pp/Ppk) are metrics that are used to quantify the overall (actual) capability of process.

Performance Ratio (Pp) signifies the process capability when process is equal to specification center accounting for overall variation. It is the ratio of specification width (USL - LSL) over process width (UCL - LCL).
Performance Index (Ppk) signifies the process capability when process mean is not equal to specification center accounting for overall variation.

Capability Indices (Cp/Cpk) are metrics that are used to quantify the within (potential) capability of process.
Capability Ratio (Cp) signifies the process capability when process is equal to specification center accounting for within variation. It is the ratio of specification width (USL - LSL) over process width (UCL - LCL).
Capability Index (Cpk) signifies the process capability when process mean is not equal to specification center accounting for within variation.

An application-oriented question on the topic along with responses can be seen below. The best answer was provided by Adil Khan on 20th Oct 2023.

Applause for the winner!

Capability Stats

Followers

October 17, 20232 yr

Q 609. Why is that we need to look at 2 capability stats - both Pp and Ppk (or both Cp and Cpk)? What are the challenges if we look at any one of them alone? Support your answers with examples

Note for website visitors -

This platform hosts two weekly questions, one on Tuesday and the other on Friday.
All previous questions can be found here: https://www.benchmarksixsigma.com/forum/lean-six-sigma-business-excellence-questions/.
To participate in the current question, please visit the forum homepage at https://www.benchmarksixsigma.com/forum/.
The question will be open until Tuesday or Friday at 5 PM Indian Standard Time, depending on the launch day.
Responses will not be visible until they are reviewed, and only non-plagiarised answers with less than 5-10% plagiarism will be approved.
If you are unsure about plagiarism, please check your answer using a plagiarism checker tool such as https://smallseotools.com/plagiarism-checker/ before submitting.
All correct answers shall be published, and the top-rated answer will be displayed first. The author will receive an honorable mention in our Business Excellence dictionary at https://www.benchmarksixsigma.com/forum/business-excellence-dictionary-glossary/ along with the related term.
Some people seem to be using AI platforms to find forum answers. This is a risky approach as AI responses are error prone as our questions are application-oriented (they are never straightforward). Have a look at this funny example - https://www.benchmarksixsigma.com/forum/topic/39458-using-ai-to-respond-to-forum-questions/
We also use an AI content detector at https://crossplag.com/ai-content-detector/. Only answers with less than 15-20% AI-generated content will be approved.

Solved by Adil Khan18

October 20, 20232 yr

Go to solution

October 20, 20232 yr

Solution

The Reason why we need to consider both Cp & Cpk or Pp & Ppk is, only then we get the complete picture where we need to actually work on to reach the targeted process capability targets.

Cp (Capability Index) or Pp (Process Performance Index) compares the process variation to the maximum allowable variation as indicated by the tolerance. Cp or Pp does not consider the process location.

Example

If we look only at Cp or Pp and do not keep a eye on Cpk or Ppk it can deceive us. (See below Image) after looking at Cp is 2.0 you may think say its world class process, is it really? Only When you see both Cp & Cpk or Pp & Ppk you realize how your process actually is.

Cp or Pp = USL-LSL / 6Sigma (sigma = Standard Deviation)

image.png.30fd3d3961490505600244a44fbc6639.png

Cpk (Capability Index) or Ppk (Process Performance Index) takes into account process location as well as process variation is derived from within subgroup range for Cpk and for Ppk it is derived from overall standard deviation.

Cpk or Ppk = Minimum [(USL - Process Mean)/ 3Sigma, (Process Mean - LSL)/3Sigma]

Example

Look at below image (only the one marked with 1 ) if you have only Ppk or Cpk, you now know your process is centered but you dont know if it is capable.

If your boss says lets make our process Ppk or Cpk = 1.67 what can you do make it even more centered? Only when you look at Cp as well as Cpk you get the complete picture.

image.png.3a85e41dee429c9a92eef5a81d3d45b5.png

If we have both Cp & Cpk or Pp & Ppk we can decide what has to be done to make our process more capable. If Cp has to be improved then process variation must be reduced and if Cpk has to be improved then process dimension must be centered.

2 yr2 yr Rohit Gandhi locked this topic

October 20, 20232 yr

This question has thrown up surprises. It seemed easy but still there are a few answers that have not been approved because they were incorrect. There are also a couple of answers that were not approved due to AI generated content.

Anyway, lets come to this question, the remaining answer from Adil Khan has been selected the best answer. However, want to add a few points

1. Looking alone at Pp (or Cp) is detrimental for two reasons

It does not consider the process centricity
It cannot be calculated for single side specifications

2. Some experienced practitioners only look at Ppk (or Cpk) value. since Ppk value will always be less than equal to Pp, therefore they work with a criteria of Ppk to be greater or equal to 1.33 (the number 1.33 could be different for some industries). This will ensure that Pp value is also acceptable. Keep in mind that process might still be off centered but is still capable of meeting customer specifications.