AQL: how a laboratory inspects a packaging lot on arrival
A truck delivers 10,000 caps. Nobody will inspect them one by one. The laboratory takes 200, counts the defects, and accepts or rejects the whole lot. The rule that sets these numbers has three letters: AQL.
By Tecmom editorial team · 2 min read
Inspecting a sample instead of the whole lot
Inspecting every part is expensive, slow, and not infallible: by the thousandth cap, the eye tires. Sampling inspection means examining a set number of randomly drawn parts, then deciding for the whole lot.
The reference standard is ISO 2859-1, which descends from a US military standard, MIL-STD-105. It provides sampling plans indexed by the AQL, the acceptable quality limit.
Reading the plan in two tables
The first table gives a code letter from the lot size and the inspection level. Level II, the most common, is normal inspection; level III is stricter, level I lighter. For a lot of 3,201 to 10,000 parts at level II, the letter is L.
The second table links that letter to the sample size, here 200 parts, and, for each AQL, two numbers: the acceptance number and the rejection number. With an AQL of 1.0, the lot is accepted if the sample contains at most 5 defective parts, and rejected from 6.
An AQL is not a guaranteed rate
The most common mistake is to read the AQL as a maximum percentage of defects in the lot. It is not. It is the quality level that a plan accepts most of the time; a slightly worse lot can pass, a slightly better one can be rejected. Sampling reduces risk, it does not remove it.
The standard also provides for switching from normal to tightened inspection after rejections, or to reduced inspection after a run of conforming lots.
Not all defects weigh the same
The specification sorts defects into three families. A critical defect makes the packaging dangerous or unusable: a cap that does not close, a missing tamper-evident band. A major defect compromises function. A minor defect affects appearance.
Each family gets its own AQL, very strict for critical defects, more lenient for appearance. These values are set between the laboratory and its supplier; the standard does not impose them. Incoming inspection complements the manufacturer's quality system, it does not replace it.
An AQL does not say that a lot is perfect. It says how far one agrees not to know everything, and that limit is negotiated before delivery, not after.
Glossary
- AQL
- Acceptable quality limit of a sampling plan.
- ISO 2859-1
- The standard for sampling plans in inspection by attributes.
- Inspection level
- The parameter that sets the sample size for a given lot size.
- Critical defect
- A defect that makes the product dangerous or unusable.