Inspection and Test
Where inspection sits
Inspection compares product to requirements by measuring, gauging, or examining; testing applies a stimulus and measures functional response. Know receiving, in-process, and final inspection, first-article inspection, source inspection, and skip-lot. Attribute inspection (go/no-go, visual) is fast and cheap but yields little information; variables inspection produces a distribution that supports capability studies and trend analysis.
Sampling plans
The classic attribute standard (ANSI/ASQ Z1.4) takes lot size, inspection level, and AQL and returns a sample size and acceptance number. Traps abound: AQL is the quality level the plan accepts most of the time (producer's risk α, typically 5%). It is not a defect target and not a guarantee. LTPD is the level the plan rejects most of the time (consumer's risk β, typically 10%). Zero-acceptance plans (c = 0) are common in aerospace and medical work but have poor discrimination at small sample sizes. Switching rules - normal, tightened, reduced - are part of the standard, and OC curves show how a plan actually performs.
Nonconforming product
A nonconformity is a requirement not met; a nonconforming unit contains one or more nonconformities. Dispositions are use-as-is (requires engineering approval), rework (to original specification), repair (to a different but functional condition, requires approval), scrap, and return to supplier. Material Review Board authority matters - production must not self-dispose.
Worked example
A lot of 1,200 at general inspection level II, AQL 1.0 gives code letter J, sample size 80, Ac = 2, Re = 3. Find two nonconforming units and the lot is accepted; find three and it is rejected.