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Mano

CP & CPK Calculation

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Hi All,

 

Could you please explain how to calculate the Cp, Cpk for the below mentioned data. its derived from the team on a daily basis quality of day. We would like to know about the capability of the process

 

Date                                  % of Quality

21-Jun-19                         100

22-Jun-19                         99.24

23-Jun-19                         99.82

24-Jun-19                         99.76

25-Jun-19                         99.82

26-Jun-19                         100

27-Jun-19                         100

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Hi Manoj

 

Cp and Cpk calculations are done for Continuous data type when both Lower and Upper Specification Limits are available.

 

Now, I am not sure how you are calculating Quality, but in a typical scenario Quality % is Discrete in nature. Even if you consider it to be continuous, you will still need both the specification limits. However, for Quality %, we usually only have the Lower Specification Limit. E.g. customer says i want at least 95% quality. Here 95% becomes the Lower Specification Limit, but there is no Upper Specification Limit.

 

For your specific case, I would recommend you to check the Binomial or Poisson capability function (depending on whether Defectives or Defects are tracked) in Minitab. 

 

Hope this helps.

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Thank you Gupta for your detailed explanations. Yes, you are absolutely right.

 

Actually I have considered the specification limits as highlighted below.

 

USL: 100.00

LSL: 99.00

Target: 99.50%

 

Further,  I have attached my calculation spreadsheet for your quick understandings. Can you please review and guide me which is right option to calculate the CP & CPK for this data type?

 

 

 

 

 

CP & CPK Calculation.xlsx

CP & CPK Calculation.xlsx

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Hi Manoj

 

The calculations seem to be right. However, one could question the USL of 100%? If it becomes the USL, then you will never strive for 100% quality :)

 

Anyways, you could verify your calculations by using our Cp, Cpk calculator at the below link as well

https://www.benchmarksixsigma.com/calculators/process-capability-calculator/

 

 

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Hi Gupta,

 

Thanks for your confirmation about the calculations and i too verified with the above link as suggested by you. Yes, the values are matches exactly.

 

Your question about the USL is really valid and realistic one:rolleyes: . For my case, can you please guide me which is the right option to measure the process capability whether we can use the data type of  "continuous" (Quality%) or "Discrete" (Defective / Defect).  

 

In my project, we are calculating the quality % with the defective mode and the sample size will be differ on a daily basis (ie: Samples data points already attached in the control chart data)

 

Your inputs would be highly appreciable to improve my understandings on this part. Thank you so much for spending your valuable time on this.

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Hi Manoj

 

If you are calculating Quality metric basis Defective calculation, you could use the Binomial Process Capability in Minitab. 

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Quote

 

Thank you so much Gupta for your inputs. Let review and get back, if required any further understanding on this.

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