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calculating CPK's

Ken379128
I was wondering if anyone has had this problem.

I run a program get my 30 pcs data send it to data page and the calculation from data page is different when done with a calculator. I got a cp 5.18 and a cpk of 5.21. I thought the cp had to be bigger? Then when I did it on my calculator I got the cp at 5.55 and the cpk at 5.52Confused
Parents
  • The suggestion above to edit the lower tolerance and make it negative of the TP tolerance won't have any effect on the Cpk (or Ppk) calculation. But it will change the value of Cp (or Pp) so that Cp will be larger than Cpk. For one sided tolerances, DataPage will only consider the +Tol to be a tolerance, and forces the Cpk (or Ppk) calculation to use that side of the tolerance. In these cases (TP, form errors, etc.) zero is not a tolerance, but is a mathematical limit. Cp (Pp) is not well defined in a one-sided tolerance, since there is not really a tolerance width. There is an option to disable computing Cp (Pp) for one-sided tolerances.



    But this doesn't seem to apply here given your tolerance band. How is it toleranced? Do you have all the tolerance on one side? It's possible DataPage is trying to treat it as a one sided tolerance if it is toleranced something like 2.246 -0, +.004. What version is Datapage?

    Also note that when you are calculating Cpk by hand, you may actually be calculating Ppk. Cpk (and Cp) use estimates of the standard deviation that come from a control chart (for xbar&r it's rbar/d2, etc). Ppk (and Pp) use the sample standard deviation in the calculation.
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  • The suggestion above to edit the lower tolerance and make it negative of the TP tolerance won't have any effect on the Cpk (or Ppk) calculation. But it will change the value of Cp (or Pp) so that Cp will be larger than Cpk. For one sided tolerances, DataPage will only consider the +Tol to be a tolerance, and forces the Cpk (or Ppk) calculation to use that side of the tolerance. In these cases (TP, form errors, etc.) zero is not a tolerance, but is a mathematical limit. Cp (Pp) is not well defined in a one-sided tolerance, since there is not really a tolerance width. There is an option to disable computing Cp (Pp) for one-sided tolerances.



    But this doesn't seem to apply here given your tolerance band. How is it toleranced? Do you have all the tolerance on one side? It's possible DataPage is trying to treat it as a one sided tolerance if it is toleranced something like 2.246 -0, +.004. What version is Datapage?

    Also note that when you are calculating Cpk by hand, you may actually be calculating Ppk. Cpk (and Cp) use estimates of the standard deviation that come from a control chart (for xbar&r it's rbar/d2, etc). Ppk (and Pp) use the sample standard deviation in the calculation.
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