Open any inspection report for your building and look for the sampling line: General Inspection Level II, a single AQL, normal severity. You’ve been there for years and the lots keep coming through. Now ask the tough question: who chose it, when, and against what risk? In most operations, the honest answer is that no one in the building today knows.
A sampling plan is an acceptance rule, not a detection rate
The published schemes, ANSI/ASQ Z1.4 and ISO 2859-1, were designed to decide whether a series of lots is produced at an acceptable quality level. They do that job well. What they do not do, and were not designed to do, is guarantee that a rare defect within a lot will be found.
One example makes it clear: a plan that takes 32 parts from a lot of 500 and accepts on zero defects. If the lot has three defective parts per thousand, the probability that the sample finds none is about 91%. The lot passed. That is not the same as saying the lot is clean.
Where Inherited Plans Come From
Almost always from one of four places: they were copied from a written validation report for another configuration, they were dragged from an earlier product family, they were taken from a customer specification for a program that has already been completed, or they were accepted as the value proposed by the software. All four are common. None of them is a justification, and justification is exactly what an auditor asks for when they ask why this plan and not another more strict one.
What Gets Lost Along the Way
The second loss is switching rules. Schemes are only statistically honest when the normal, strict, and reduced inspection moves as defined: strict after a certain number of rejected lots, back to normal after a defined streak, and reduced only under specific conditions. In practice, plants remain in normal indefinitely. Tables are used; the mechanism that gives them their protective power is abandoned.
The third is characteristic classification. Critical, major, and minor characteristics should not share the same acceptance number. Applying a single AQL to an entire drawing means accepting the same statistical risk on a characteristic that affects function as on one that affects appearance. That is a decision, and it should be made deliberately and in writing.
What a Defensible Plan Includes
None of this is an argument for inspecting more. It is an argument for being able to explain the plan that is being used. A defensible sampling scheme has:
- A documented justification linking each class of characteristic with an inspection level and an acceptance number.
- The lot size ranges to which it applies.
- The switching rules and who is responsible for applying them.
- A per-lot record showing what was actually sampled.
In an audit, the plan is only as good as the evidence that it was followed exactly as written.
What Disciplined Sampling Looks Like on the Floor
- The plan is executed as written. Sample size versus actual lot size, truly random selection and number of acceptances applied without negotiation at the end of the shift, including lots that arrive late.
- A record that holds up in an audit. Per lot: identification, sample size, characteristics inspected, results, disposition, inspector, date, and time.
- Switching rules that actually switch. Switching between normal, tightened, and reduced inspection controlled against your criteria, with notice as soon as a series of lots triggers a switch.
- Inspectors trained before the first lot. Against your criteria, your limit samples, and your defect classification.
When sampling shows that a lot cannot be released, the next step is usually incoming inspection or 100% sorting.
About our scope: RIDYS Group provides inspection, sorting, rework, and on-site technical personnel for components, devices, and packaging. We do not hold ISO 13485 certification or registration under 21 CFR Part 820. Our personnel work within your quality system, follow your procedures and sampling instructions, and product release decisions are entirely yours.
Did You Inherit a Plan No One Can Explain? Send us the sampling line from one of your work instructions and we’ll tell you what it assumes and what it doesn’t cover.
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