[Wayne Taylor]

Two issues occur with destructive testing, First is the cost of testing. With destructive testing, the cost of testing is at least as great as the cost of producing the unit. However, there are cases involving nondestructive testing where the cost of testing exceeds the cost of the unit. As the cost of testing increases, economics dictate that the sample sizes be reduced lessening the protection.

The second issue is the fact that rejected lots cannot be rectified by 100% inspection. For destructive testing, the choice is between release and discarding (or recycling or downgrading). However, again there are cases involving nondestructive testing where the cost of performing a 100% inspection is more expensive than the potential value of the lot so that 100% inspection is not a viable option.

I have attempted to make the case that it is not two distinct situations, destructive versus nondestructive testing, but really a continuum from inexpensive testing to very expensive testing. The impact of this is that defects types are frequently segregated not only by severity (critical, major, minor) but also by cost of testing (visual, functional) and inspected using different sampling plans. For example, major visuals might be assigned an AQL of 0.25% while major functionals might be assigned an AQL of 1.0%. Since major visuals and major functionals are of the same consequence but major visuals are easier to rectify, the AQL is set lower so that one is quicker to reject
them.

More on selecting sampling plans based on economics can be found in Chapter 4 of my book Guide to Acceptance Sampling. One concept, I mention in the article Classifying Defects and Selecting AQLs is that of break-even quality. The formulas I give there are for the case of nondestructive testing only. Chapter 4 gives break even qualities for the case when 100% inspection is not possible.

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