AQL Sampling in Product Inspection, Explained
How acceptance sampling decides whether a shipment passes or fails, and how to read an inspection report with confidence
When an inspector checks a lot of 5,000 units, they do not open all 5,000. They draw a random sample, classify the defects they find and compare the count with an acceptance number. That decision rule is based on AQL, the Acceptance Quality Limit, and on the sampling tables of ISO 2859-1 (equivalent to ANSI/ASQ Z1.4).
AQL is used in almost every pre-shipment and final random inspection, yet it is widely misunderstood. Buyers often think “AQL 2.5” means “2.5% defects are allowed in my order”. This article explains what AQL really means and how to use it well.
1. What AQL means
The Acceptance Quality Limit is the worst average quality level, expressed as percent defective or defects per hundred units, that is considered acceptable over a continuing series of lots. It is a property of the sampling plan, not a promise about a single shipment.
In practice, AQL answers one question: how many defects can be found in the sample before the lot is rejected?
2. Classifying defects
| Class | Definition | Typical AQL |
|---|---|---|
| Critical | Could cause harm to the user or breach a legal or safety requirement | 0 (no critical defect accepted) |
| Major | Likely to cause failure, reduce usability or lead to a complaint or return | 2.5 |
| Minor | Departs from the specification but is unlikely to affect use | 4.0 |
These values are common for consumer goods, but they are agreed between buyer and supplier and should be stated in the purchase order or inspection checklist.
3. From lot size to sample size
The sampling plan depends on:
- Lot size: the number of units in the shipment.
- Inspection level: General Level II is the default. Level I gives smaller samples; Level III gives larger samples. Special levels S-1 to S-4 are used for costly or destructive tests.
- AQL for each defect class.
From the lot size and inspection level, the table gives a code letter, which sets the sample size. The AQL then sets the acceptance number (Ac) and rejection number (Re).
Worked example
A lot of 5,000 units, General Inspection Level II, single sampling, normal inspection:
| Step | Result |
|---|---|
| Code letter | L |
| Sample size | 200 units |
| Major defects, AQL 2.5 | Accept if 10 or fewer; reject if 11 or more |
| Minor defects, AQL 4.0 | Accept if 14 or fewer; reject if 15 or more |
| Critical defects | Reject if any is found |
If the inspector finds 8 major and 16 minor defects, the lot passes for majors but fails for minors, so the overall result is fail, unless the buyer decides otherwise.
4. What a good inspection includes besides AQL counting
- Quantity and packing check: cartons, units per carton, shipping marks.
- Visual and workmanship check on the AQL sample.
- Measurements and functional tests against the specification, usually on a smaller sample.
- Labelling and marking, including legal and language requirements of the destination country.
- Specific tests agreed in advance, such as drop tests, moisture, weight or barcode readability.
- Photos of each defect class and of packing.
- Sampling for laboratory testing where the specification requires it.
5. How to use AQL well
- State the AQL and inspection level in the purchase order, not only in the inspection request.
- Write a clear defect list with examples of critical, major and minor defects for your product.
- Use a random sample drawn from across the lot, not cartons chosen by the factory.
- Treat a pass as a pass for that sample, not as proof that the lot contains no defects.
- Track results over time: repeated failures from a supplier call for a supplier audit and root cause analysis, not just re-inspection.
6. Inspection stages
| Stage | When | Purpose |
|---|---|---|
| Pre-production inspection (PPI) | Before mass production | Check materials, components and the approved sample |
| During production inspection (DUPRO) | 10–60% produced | Catch problems while they can still be corrected |
| Final random inspection (FRI) | At least 80% packed | Decide whether the lot can be shipped |
| Container loading supervision | During loading | Check quantity, condition and sealing |
Conclusion
AQL is a decision rule, not a defect allowance. Used with a clear defect list, the right inspection level and a truly random sample, it gives buyers a fair, repeatable basis for accepting or rejecting a shipment before they pay for it.
Check your last purchase order: does it state the AQL for critical, major and minor defects?