Capability and sampling

Six Sigma – iSixSigma Forums Old Forums General Capability and sampling

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    I am trying to estimate the capability of a production process. The data from the machines, who are producing bags with shredded cheese, are sent to a computer, where I get the data. The data consists of mean weight from subsamples, the standard deviation within the subsamples and the number og products within the subsamples (typically between 500 and 1000 items).Here’s my problem:I’ve calculated Ppk ((grand mean-LSL)/3*the mean of the standard deviations from the subsamples) and I come up with a Ppk of 0,794. Acoring to several conversion tables I’ve read, it should equate to an expected quality level of somewhere between 90-95%. But I still have not registered one single sample with a quality-level below 98%.Why such a big difference?ThanksPeterI’ve followed “Six Sigma and the Quality toolbox”, or at least tried to:)

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