# link between confidence interval Ppk

Six Sigma – iSixSigma Forums Old Forums General link between confidence interval Ppk

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• #50738

Sorour
Participant

Hi
Is it possible to calculate the Ppk for a corresponding confidence interval and sample size?
For example for a confidence interval of 95/99.9 the Ppk for 30samples is 1.34 (using aql)

Are there tables or a calculation I can use to develop Ppk for 95/99.9 for different sample sizes and different confidence intervals? E.g. 9 17 21 etc.

Thank you
Paul

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#174832

Ron
Member

Ppk is a long term process capability typically with 100 samples taken or more.
Confidence Intervals are the probability that a specifc value will fall within this range.
Ppk is very specfic  Confidence intervals are general.

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#174833

The Kid
Member

Hi Paul,
I am not 100% sure about this, but I think some time ago I did read some articles describing this. I think the outcome of the study was a look up table with sample size and confidence interval for CPK values.
So I would suggest you do some googling on the same. Would be great if forum experts can provide some insight on this.
Cheers,
The kid

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#174843

Sorour
Participant

Hi Guys.
I was talking to a work colleague. He said you can determine the Ppk by using your confidence intervals and k value. Simply by using the confidence formula 95/999 = mean k X sd and LSL- mean divided by 3 x sd = Ppk you can work out that the Ppk is 1/3 the k value.
Dont know if anybody heard this before or if there some data behind it. I would be interested to hear your comments.

Secondly, typically what would be the smallest sample size you would use to work out Cpk/Ppk? I am recommending a minimum of 15 but ideally 30.
Thanks again
Very much appreciated
Paul

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#174912

Mikel
Member

Pp can be related to the tolerance interval which is a containment interval of individual values at a given level of confidence. E.g., a 95/95 tolerance interval contains 95% of the expected values at a 95% confidence level. Tolerance Intervals are sensitive to departures from normality so transformation is required of most data.

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