How to calculate Cpk in this case ?

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• #28415

Pai
Participant

How to calculate the Cpk when the specified limit is of type (say) 0.1mm max., because in this case lower value means better quality & we do not have any mean.

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#70720

Sridhar
Member

Hi rajesh,
Why dont you have a mean you. What ever the data points are there then definitely you have a mean. You will not have lower specification limit.
You can calculate Cpk as (USL – mean / 3*Sigma)
sridhar
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#70724

TCJ
Member

Are you using the right analysis?  Why do you need to calculate Cpk?  I do not know the details of your problem, but it sounds like you could have attribute data.  If this is the case you can calculate DPMO directly or use a binomial process capability analysis.

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#70729

Member

This is a typical case observed and we call this as the process is bounded at zero.The data infact in such a case may not even be normal.If the data is not normal various diffrent approaches will have to be used.Direct calculation of Cpk may be wrong.For further information on haow to handle such processes pls feel free to contact me at [email protected]

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#70752

Participant

Hi Rajesh:
This is the typical case where directly calculating Cpk is quite complex. Generally when you have one sided tolerance, the data doesnot follow normal distribution. In most of the case it follows poisson distribution. What you do, is calcuate DPMO and from there you calcuate the sigma of the process. Sigma and Cpk can be transformed.
Jugal

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#70756

Gautam
Participant

Rajesh,
May be you have an attribute data…say ‘If  ‘X’ < 0.1mm, it is good, else bad".
Calculate DPU  and identify number of opportunities. Convert DPU to DPMO.  This would give you an idea of the process capability.

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#70782

Mike Carnell
Participant

I hope you intend to respond to some of these questions. At this point you have not provided enough information to have your questions answered adaquately. The place to start is where TCJ started. What type of data do you have?

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#71773

Jatinder Singh
Participant

The data here is of variable type. Upper specified limit is 0.9 Max. Least count of the measuring instrument is 0.01 . As you know from above info. it is clear that lesser the value better is the quality & ( theoretically zero means best quality ) but actually we never get zero .

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#71777

Mandeep vats
Participant

The besway is to look for FORD SPC manual (not the QS9000 reference manual) But the original FORD SPC manual and you will get the exact method and the calculation for finding the Cpk.Any queries feel free to contact me.
regards
Mandeep vats

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