DPPM, DPMO are meaningless, as has been discussed in other threads. I can explain further if you wish. These terms should not be used.
I disagree that DPPM and DPMO are meaningless, any more than percentages are meaningless. If you make 25 parts and one is bad, then you have 4% defective, which is the same mathematically as 40,000 DPPM. 4% doesn't imply you made 100 parts and 4 were bad, anymore than 40,000 DPPM implies that you made 1,000,000 parts.
What is meaningless (and I expect this is what you mean) is the extrapolations made from a small number of parts and assuming a particular distribution (i.e. normal distribution). For example "6 sigma = 3.4 DPPM" is meaningless. The sigma level itself is just an estimate based on an observed mean and st dev; the extrapolation assuming a normal distribution is even more suspect!
"About the only place that this procedure will help is in trying to determine which batches have already been screened and which batches are raw, unscreened, run-of-the-mill bad stuff from your supplier. I taught these techniques for years but have repented of this error in judgment. The only appropriate levels of inspection are all or none. Anything else is just playing roulette with the product."
This seemed self-contradictory for a moment -- giving a use for sampling, but then saying there was no use

. Then I realized what Wheeler was saying - sampling is useful for SCREENING, but useless for CONTROLLING or IMPROVING. I seem to be mirroring Wheeler, only a decade later. I got into the theory of sampling but then realized how limited it actually is for detecting small shifts.
Tim F