The Deming Inspection Criterion for Choosing Zero or 100 Percent Inspection

Abstract

W. E. Deming has derived a statistical criterion for performing incoming inspection or inspection at some intermediate step of a process. The criterion equation indicates that either 100% inspection or no inspection at all are the proper responses, depending on the magnitude of the expression for the criterion. One critical assumption is that the process is being monitored continuously by control charts, and that material arriving at the inspection station does not include any batches made while the process is out-of-control. Deming's statistical equations referring to incoming inspection and to inspection between steps of a process result in the same expression for the criterion when the fraction nonconforming is small. Several examples from manufacturing experience are analyzed to show how this criterion works in practice. The examples show that practice follows the Deming Inspection Criterion in the specific cases cited, and that cost savings and cost avoidance have resulted from 100% inspection in the cases in which the Deming Inspection Criterion indicated that 100% inspection was the proper response for the particular conditions. The goal, according to Dr. Deming, is to achieve process improvements through statistical control to a degree permitting the cessation of 100% inspection. However, in several of the examples this goal was not achieved over a protracted period of time, and 100% inspection remains necessary indefinitely.

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