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Record what a sample inspection found, classify each defect, and decide the lot against the acceptance numbers of your sampling plan.
ON THE EXAM · CQI III.B.4, III.C.5, CQT IV.A.3, IV.C.1 · FREE · NOTHING YOU TYPE LEAVES YOUR BROWSER
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Classifying defects by seriousness
Classification puts the inspection effort and the acceptance criteria where the consequences are. Sampling standards such as ANSI/ASQ Z1.4 and common practice use three classes, described here in plain terms:
- Critical: a defect that could make the product unsafe or hazardous for the people who use or service it, or that would stop a major end item from doing its job.
- Major: not critical, but could cause the product to fail or make it noticeably less fit for its intended purpose.
- Minor: a departure from the specification that does not materially affect how the product is used or how well it works.
The classification belongs in the inspection plan, decided in advance, not by the inspector at the bench.
Defects and defectives
A defect (nonconformity) is one failure to meet a requirement. A defective (nonconforming unit) is a unit with one or more defects. A closure with flash and a scuff is two defects but one defective. Under Z1.4 definitions a unit is a defective of the most serious class it contains, so a unit with a major and a minor defect is a major defective. A plan states whether its acceptance numbers count defectives (percent nonconforming) or defects (nonconformities per hundred units).
Accepting or rejecting the lot
Each class has its own acceptance number Ac and rejection number Re. If the count is at or below Ac, the class accepts; at or above Re, it rejects. The lot is accepted only if every class accepts. Critical defects are usually set at zero acceptance, often with 100% inspection for that characteristic.
Rejecting a lot does not decide what happens to it. The lot is identified and segregated, and the disposition (rework, sort, use as is, return or scrap) is made through the nonconforming material process, often by a material review board.
Reading the result
The table shows defects and defective units for each class against Ac and Re, plus defects per unit (DPU) and defects per hundred units. The checks flag a missing plan, unclassified defects, a sample larger than the lot, and the switching-rule consequence of a rejection.
On the CQI exam
Expect to classify a described defect, to tell defects from defectives, and to apply Ac and Re for each class. A frequent trap is counting a unit twice because it has two defects of different classes.