# Talking in terms of 6 sigma, why 4 ppm would be very costlier than 1 ppm (part per milion)?

In industrial manufacturing lines, there is a a limit on the number of discarded parts per million.I need to have a sense about this, e.g. How different is 4 ppm from 1 ppm and how much does it indicate the improve in production quality?

To me considering the price of a part as 1€, 1 ppm and 4 ppm seem so trivial

• Hi - I have read your question, and there is not sufficient information to provide an answer. Whether the difference is a problem at all depends on so many things, and what constitutes "production quality" needs to be defined. For example, if you make your inspection process tighter and then start scrapping 100ppm, this could mean that production quality goes up - i.e. the parts coming off the line are higher quality. Or, if you have the wrong settings on a maching upstream, and scrap 100ppm, you could have lower production quality. Your question, as it stands, is a bad question. – Jonathan R Swift Sep 27 '19 at 9:57
• Fabio - Where did you get the impression that someone said this is a bad question? It could certainly afford more context and explanation, but it's not inherently bad. Downvotes don't mean a question is bad, they mean either a) insufficient research effort shown, or b) unclear / needs more detail, or c) not useful for the site's focus. – user16 Sep 27 '19 at 11:20
• @GlenH7 Thank you Geln ,well two comments higher, it's indicated by Jonatan as a bad question. yet , I am new in this field, and I would lack reasoning the way Jonatan did. what I was searching for, is in Jonathan's comment. 1 ppm is the lowest I guess. but if it was first 100 ppm and 1 ppm was asked, how would I reason against that to say that's too high an expectation.. – FabioSpaghetti Sep 27 '19 at 11:41
• If we talk about quality and 6-sigma (which roughly means 1 ppm defects), the 1 ppm refers to test escapes, not test rejects. Even if a part only costs \$1, if testing fails to catch a bad part and we accidentally send it to a customer, where it fails in the field, the eventual cost (in failure analysis, corrective actions, documentation, time spent communicating with the customer, etc) will be much, much more than \$1. – The Photon Sep 28 '19 at 16:00
• dear @ThePhoton please be so kind to me and post your great comment as an answer – FabioSpaghetti Oct 5 '19 at 17:24

If we talk about quality and 6-sigma (which roughly means 1 ppm defects), the 1 ppm refers to test escapes, not test rejects.

Even if a part only costs \$1, if testing fails to catch a bad part and we accidentally send it to a customer, where it fails in the field, the eventual cost (in failure analysis, corrective actions, documentation, time spent communicating with the customer, etc) can be much, much more than \$1.

There is also a benefit to demanding 6-sigma (1 ppm defects) or better quality from your vendors.

If you buy a \$1 part, and assemble it into a \$100 (or \\$1000) product, where it causes the whole product to fail, then again the cost of that defect is much higher than the cost of the faulty part itself.

• I appreciate your understanding and profession ! – FabioSpaghetti Oct 5 '19 at 17:31
• I would also add one more thing that I noticed later, if your batch is like 100,000 , then having 55 parts escaped, means 550 ppm, so it's so hard to keep that level to 4 ppm – FabioSpaghetti Oct 5 '19 at 17:33

1ppm is 4 times better than 4ppm. If 4ppm of parts are indicated it means that 0.0004% of these parts have to be discarded.

It is always better to have a lower number of discarded parts which means that 0.0001% of defects is better than having 4 times more. Lower ppm (in this case) means less costs and higher productivity.

• This does answer your question "how different is 4ppm from 1ppm"? Exactly 4 times better. – Jonathan R Swift Sep 27 '19 at 9:16
• @FabioSpaghetti don’t make the error of assuming the identity of who downvoted... – Solar Mike Sep 27 '19 at 13:21
• There is nothing in the question that is about standards for accepting or rejecting any particular value. People can only answer the question you asked, not the one you were thinking about but didn't write down. (And the question you asked is completely trivial, unless you don't understand the concepts of "one" and "four"). – alephzero Sep 27 '19 at 16:03
• The FDA has standards which require purity to better than one part per billion, not one part per million. – alephzero Sep 27 '19 at 17:46
• @FabioSpaghetti I have spent time answering some of your previous questions, and you have deleted the question (and therefore my answer) literally seconds after I posted it. I assume the reason was that you don't want anyone else to read the answer, or even to know that you got the answer here. That is a common trick played by people wanting to "cheat" on tests or homework. If you want to behave that way, don't expect any "niceness" from me in return. – alephzero Sep 27 '19 at 17:49