Buyers new to third-party inspection often assume the inspector checks everything. Nobody checks everything. Checking 5,000 chairs one by one would cost more than the chairs. AQL sampling is the compromise that makes inspection affordable, and understanding it is the difference between a specification that protects you and one that does not.
What AQL actually is
AQL stands for Acceptable Quality Limit. It is defined in ISO 2859-1 (and its American twin, ANSI/ASQ Z1.4). It answers two questions:
- How many units should be drawn from a batch of this size?
- How many defective units can be found in that sample before the whole batch is rejected?
It is a statistical bargain. You accept that some defects will reach you, in exchange for an inspection that costs hundreds rather than thousands.
Inspection levels
There are three general levels. Level II is the commercial default and is what you should specify unless you have a reason not to.
- Level I — a smaller sample, less confidence, cheaper. Use for low-risk repeat orders from a supplier with a long clean record.
- Level II — the standard. Use this.
- Level III — a larger sample, more confidence, more expensive. Use for a first order from a new supplier, a high-value product, or after a failed batch.
Sample sizes at Level II
| Batch size | Sample drawn |
|---|---|
| 151 – 280 | 32 |
| 281 – 500 | 50 |
| 501 – 1,200 | 80 |
| 1,201 – 3,200 | 125 |
| 3,201 – 10,000 | 200 |
| 10,001 – 35,000 | 315 |
Note what this means: for an order of 10,000 units, 200 are checked. That is two per cent. The statistics are sound, but you should hold the correct expectation — this is a batch-level verdict, not a guarantee about every unit.
Defect classes
Defects are sorted into three classes, and you set a separate limit for each.
- Critical — makes the product unsafe or illegal to sell. Set this to zero. Always. One critical defect fails the batch.
- Major — likely to result in the product being returned, rejected or unusable for its purpose. Typical limit: 2.5.
- Minor — a departure from specification that most end customers would accept. Typical limit: 4.0.
The number is not a count. AQL 2.5 does not mean “2.5 defects allowed”. It refers to a tolerance percentage that maps, via the standard’s tables, to an accept/reject number for your specific sample size.
Accept and reject numbers
| Batch | Sample | Accept (2.5 major) | Reject |
|---|---|---|---|
| 281 – 500 | 50 | 3 | 4 |
| 501 – 1,200 | 80 | 5 | 6 |
| 1,201 – 3,200 | 125 | 7 | 8 |
| 3,201 – 10,000 | 200 | 10 | 11 |
So a 2,000-unit order at Level II, 2.5 major: 125 units are drawn. Seven majors and the batch passes. Eight and it fails.
The mistake that makes all of this useless
Agreeing the AQL level after the defects are on the table.
If your purchase order is silent on sampling, then when the inspector finds nine major defects in a sample of 125, the conversation becomes a negotiation about what should count as major, and about whether nine is really so many. The factory will argue, the vessel is booked, and you will accept.
Fix the level, the classes and the accept numbers in the purchase order, before production. Then the inspection produces an answer instead of an argument.
Writing the defect definitions
The other half of the work — and the half buyers usually skip — is defining what counts as major for your product. “Scratches” is not a definition. “Any visible scratch longer than 10 mm on a surface visible in normal use” is.
Spend an hour producing a one-page defect classification for your product, with photographs if you can. Attach it to the purchase order. It will be the most useful page in the file, and it makes every subsequent inspection faster and less arguable.
When AQL is the wrong tool
Sampling assumes the batch is homogeneous. If your order is 12 SKUs made on different lines over three weeks, treating it as one batch will hide a systematic problem in one SKU inside an acceptable overall number. Inspect by SKU, or at minimum instruct the inspector to draw proportionally and report by SKU.
Sampling also cannot substitute for testing. Chemical compliance, load-bearing capacity, flammability and electrical safety are laboratory questions. An inspector can pull a sample and send it to an accredited lab; they cannot answer it on the factory floor, and any inspector who says otherwise is telling you something about their standards.