AQL Sampling: What a Pass Really Means for Buyers

“Passed AQL” is a useful inspection result only when the lot, plan, sample and defect rules are visible. It is a decision made from partial information, not a guarantee about every unit in the shipment.

AQL is not sample arithmetic

The acceptance number is not calculated by multiplying the AQL value by the sample size. AQL indexes an acceptance-sampling scheme whose sample size and acceptance rule produce an operating-characteristic curve across different underlying lot qualities.

Both parties carry decision risk

NIST describes producer's risk as the chance of rejecting a lot at an acceptable process quality and consumer's risk as the chance of accepting a poor lot. A passed sample therefore does not prove that every uninspected unit conforms, while a rejected sample does not mathematically prove that every unit is defective.

Freeze six inputs before inspection

Define the lot and SKU split; the standard and version; the inspection level or other plan parameters; the random-selection method; the written specification and defect definitions; and the acceptance/rejection rule plus failed-lot disposition. Without these inputs, the label “AQL” is incomplete.

Keep the specification separate

The product specification tells the inspector what counts as conforming. The sampling plan tells the buyer how observations from the selected sample support a lot decision. Sampling cannot repair vague workmanship criteria, biased unit selection or an uncontrolled reference sample.

Let product risk select the control

Do not copy a familiar AQL setting into every purchase order. Safety-critical features, destructive tests, small lots, unstable processes or high consequences may justify tighter, special, 100% or different controls designed by competent quality personnel.

Primary sources

• ISO — ISO 28590 introduction to the ISO 2859 series
• ISO — ISO 2859-4:2020 online preview
• NIST/SEMATECH — Lot acceptance sampling plans
• NIST/SEMATECH — Operating characteristic curves

Use these controls as a procurement framework, not as product-specific engineering, legal, regulatory, certification or warranty advice. Verify the actual supplier, product, transaction and jurisdiction with qualified professionals.

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