I work for an automotive supplier and our QA dept routinely uses a sample size of 32 for CpK data. I've asked several people what the basis for this number is but nobody has been able to tell me, just that's what we've always done. It seems to me that there would have to be some consideration of the qty of parts in question. If we machine say 100,000 aluminum bodies in a month, would taking 32 pieces from one of the machines really tell you what the capability for that machine to hit that dimension is? Could anyone shed some light on this for me? Anybody know where 32 comes from? More generally, how can you choose a sample size? If, say, you have a sample of 60,000 pcs that may have some defect, how can you pick a qty to measure to gage the number of defects and how large (dimensional variation) the defect may encompass? Thanks in advance for your thoughts.
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