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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