AQL Explained: How Pre-Shipment Inspection Sampling Works

When you order 5,000 units from a factory, nobody checks all 5,000. An inspector pulls a statistically chosen sample, counts defects, and compares the result with limits you agreed in advance. AQL is the system that sets those numbers.

The three inputs#

An AQL inspection plan needs three things:

1. Lot size — how many units are in the shipment.
2. Inspection level — how large the sample is relative to the lot. General Inspection Level II is the default for most consumer goods; Level I is lighter, Level III stricter.
3. AQL values — one per defect class.

From these, the ISO 2859-1 tables give a sample size and, for each defect class, an accept number (Ac) and reject number (Re).

Defect classes#

Stricter buyers — for example in cosmetics or children's products — often use 1.5 or 1.0 for major defects.

Worked example#

For a lot of 5,000 units at General Inspection Level II, the tables give sample size code letter L, which means 200 units are checked. At AQL 2.5 for major defects, the lot passes with up to 10 major defects and fails at 11. At AQL 4.0 for minor defects, up to 14 are accepted and 15 fails.

Choosing the inspection level#

The inspection level changes how many units are checked, not how many defects are allowed per unit checked.

• General Inspection Level I takes a smaller sample. It suits repeat orders from a supplier with a clean track record, or low-value goods where a larger sample costs more than the risk.
• General Inspection Level II is the default most buyers and inspection companies use when nothing else is agreed.
• General Inspection Level III takes a larger sample. Use it for a new supplier, a product that failed before, or goods where a bad lot would be expensive to fix after arrival.
• Special levels S-1 to S-4 use very small samples. They are meant for destructive or time-consuming tests, such as measuring every seam or running a product until it fails, not for the general visual check.

ISO 2859-1 also has switching rules. After a run of passed lots you may move to reduced inspection, and after failures you move to tightened inspection with stricter accept numbers. Few small importers apply these formally, but the idea is useful: inspect harder when a supplier's recent lots have failed.

What AQL does not cover#

AQL counts defects in a random sample. It does not replace:

• Functional and safety tests (drop tests, electrical safety, lab testing for chemicals).
• Checks on packaging, labels and cartons, which should be on your inspection checklist.
• Production monitoring — an inspection at 80–100% packed is the last chance to fix problems, not the first.

For the full list of what an inspector should check besides the AQL count, see the pre-shipment inspection checklist. If defects still reach you, what to do when goods arrive defective covers evidence, claims and negotiation.