Capability - CPk comparison values

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Welshwizard

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#12
Hi,

Another angle on your dilemma.

By asking for a Cp your customer by implication wants you to demonstrate that you have "elbow room" such that the natural limits of your process fit inside the tolerance width.

You know and hopefully your customer does too that in order to state your Cp your process has to be stable....in statistical control. You also know that if it's not stable at least with the data you have you can't say the past (as for all data analysis) can be extrapolated into the future and therefore the question of Cp will be moot.

So, if your process is in statistical control, preferably using as many samples as you can get you're hands on you can state an interim Cp in the knowledge that it will solidify and even get better in the future as long as the process remains stable. Of course you would have to agree this with your customer but it's better than stating a Cp where the underlying data is not stable and no one is making any effort to fix the issue, that's just hypothetical.

Cheers
 

Mike S.

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#13
Once you reach 10 data points, consider creating an I-MR control chart. I can't find the reference at this moment, but I recall reading you can start to calculate reasonable limits once you get to 10 data points. See if your resulting control limits are within the spec limits. It would seem more valuable to me than Cpk. JMO.
 
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