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Economically-optimal inspection policies

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Deming (1982) reiterated an observation that had already been made by Mood (1943) and Barnard (1954). For a stable process, in the absence of any important sources of batch-to-batch variation, the only possible economically optimal sampling plans are either "no inspection" or "100% inspection". Where there is batch-to-batch variation , a case can be made for acceptance sampling of the batches, as discussed in Barlow & Zhang (1984), Vardeman (1986) and Vander Wiel & Vardeman (1994).

The implications of 100% inspection are:

• Escalated manufacturing costs (leading to loss of market)
• Increased through-put time (leading to increased inventory, WIP, costs and loss of market)
• Impractical for destructive testing
• Inspection that is not 100% reliable is not 100% inspection
• Loss of ownership of quality by production ("inspection will catch it")

All these considerations make it desirable to avoid 100% inspection wherever possible.

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This page last updated 13^th July 2001

Copyright ©2001 A N Cutler