sigma

engineering partnership

Meaning of "defective"

"Not everything that can be counted counts. Not everything that counts can be counted."

Albert Einstein

Inspection involves the scrutiny of an item and its classification into "conforming" or "non-conforming" (or "defective"). There are two common routes to this:

1. Measure some physical parameter and compare with a standard specification limit
2. Scrutinise the item and subjectively compare it against a standard. The standard can be in the form of a description with or without a visual aid (VA).

There are two critical questions to ask about any inspection process:

1. Are we measuring/ scrutinising the right characteristic?
2. Have we set the boundary between conforming and non-conforming in the right place?

The answer to question (1) is usually established through quality-function deployment (QFD) as described in Mizuno & Akao (1994). The need for question (2) was challenged by the Japanese statistician Genechi Taguchi who observed that design engineers would really like to have all the items manufactured exactly on the nominal value for a characteristic. Taguchi argued that every departure from nominal entailed some "on-cost" to society, and hence, ultimately, to the originating organisation. The cost might be in terms of early wear-out, effort spent in selective assembly, extra material used, additional machining time or the need to design-in a safety margin. The economically optimal place to be is On Target, Minimum Variation (OTMV). Taguchi promoted a suggestion made by the economists John von Neumann and Oskar Morgenstern in the 1940s that this could be represented by a "loss function" that grows increasingly rapidly as the item diverges from nominal. Taguchi also investigated the cases where the characteristic is of the "bigger the better" or "smaller the better" type.

There is an implied loss-function within the conventional model of inspection. Inspection suggests that everything between the specification limits is just fine (zero cost) and that everything outside is catastrophic (unacceptable cost).

Don Wheeler pointed out that the area between the specification limits really says "deny losses exist" and that outside, "losses too great to deny."

When we set a specification limit, we define the limit beyond which losses cannot be tolerated but do not, necessarily, minimise total economic loss in the system.

Home page of Inspection and Quality Control

sigma   HOME

engineering partnership

This page last updated 13^th July 2001

Copyright ©2001 A N Cutler