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The Parts-Per-Million Problem

When does a computation result in a number?

Donald J. Wheeler
Mon, 05/11/2015 - 10:56
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Parts per million (ppm) is part of the language of Six Sigma. It pervades the sales pitch and is used in all sorts of computations as a measure of quality. Yet what are the rules of arithmetic and statistics that govern the computation and usage of parts per million? To discover the answers read on.

Reporting quality levels

The emphasis upon parts per million has generated some confusion in practice. I remember one factory visit where they told me they were “doing Six Sigma.” As we went around the plant, each department had a large sign hanging from the rafters that showed the department output during the past month along with the parts per million nonconforming level achieved. I didn’t have to actually ask how the Six-Sigma program was working—the values spoke for themselves. Every parts-per-million value in the plant was greater than 30,000! Say a department produced 7,555 parts and had 368 nonconforming. While they reported this as 48,709 parts per million nonconforming, at the end of the month it was still 4.9 percent bad product! When your nonconforming product is at the parts-per-hundred level, nothing is gained by adding four extra digits and reporting parts per million nonconforming.

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Comments

Submitted by Dr Burns on Mon, 05/11/2015 - 13:28

SS hype

An excellent article as always.  However no amount of rational discussion is likely to slow the SS religion.  As I pointed out a decade ago there is a wonderful parallel with the religion of man caused global warming.  Despite no evidence of any kind to support the theory, half the population follows blindly.  Even now 2 decades after the (all natural) warming from the Little Ice Age stopped, people still believe in the fantasy.  Religions such as Six Sigma and global warming, run on hype, not science.  The hype will continue as long as there are those milking the madness for millions.  It will take more than articles like Don's to bring back common sense.

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Submitted by Dr Burns on Wed, 05/13/2015 - 13:07

In reply to SS hype by Dr Burns

I wonder if the lack of

I wonder if the lack of comments is a sign that the cringe worthy dpmo is dying and that the religion of SS and other specification based methodologies are being buried?

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