JMP Users – How do you calc Cpk?

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    Ken K.

    I’m wondering how JMP users calculate the newer Cpk index that uses the pooled standard deviation from a control chart.
    To my knowledge, JMP only calculates the capability index that uses the sample standard deviation, and gives the user the option of including the label (Ppk).



    I found the same issue with JMP, as it only uses sample std. dev.  I did an in depth comparison with the output from Minitab, manual calculations, and JMP output to verfiy this.
    Regardless, if you compare the result obtained from the Cpk v. Ppk formula (the difference being in the denominator – std. dev. value), the results are very similar.  What really has an impact on your capability value is how you conduct the sampling:  all samples conducted within a short time period versus a long-term sampling plan involving multiple lots of raw material, multiple shifts and operators, etc.
    I wouldn’t focus on how JMP calculates Cpk, as the results are very similar regardless of formula used.

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