AQL stands for Acceptance Quality Limit (older texts say Acceptable Quality Level). In garment inspection, it is used to decide how many pieces to check from a shipment lot and how many defective pieces can be found before the lot is rejected.
AQL sampling is described in ISO 2859-1 and in older equivalent tables such as ANSI/ASQ Z1.4 and MIL-STD-105E. Most buyer inspection manuals use sampling plans from this family of tables.
The main inputs#
- Lot size: the number of pieces in the lot being inspected.
- Inspection level: how much sampling is done relative to lot size. General levels are I, II and III; level II is the usual default. Special levels (S-1 to S-4) are used for small samples, such as some measurement checks.
- AQL value for each defect class: for example 2.5 for major defects and 4.0 for minor defects. Many buyers do not allow any critical defects.
- Inspection severity: normal, tightened or reduced. Normal inspection is the usual start.
How the table works#
- Find the code letter from the lot size and inspection level.
- Use the code letter to find the sample size.
- For each AQL value, read the accept number (Ac) and reject number (Re).
If the number of defective pieces found is at or below Ac, the lot is accepted for that defect class. If it is at or above Re, the lot is rejected.
Worked example#
A lot of 3,000 polo shirts is inspected at general level II, single sampling, normal inspection, with AQL 2.5 for major defects and 4.0 for minor defects.
- Lot size 3,000 at level II gives code letter K.
- Code letter K gives a sample size of 125.
- For AQL 2.5: Ac 7, Re 8.
- For AQL 4.0: Ac 10, Re 11.
The inspector checks 125 shirts. If 8 or more have major defects, the lot fails, whatever the minor count. If there are 7 or fewer major defective pieces and 10 or fewer minor, the lot passes.
The AQL sample size calculator gives code letters, sample sizes and accept/reject numbers for general levels I to III.
Defect classes#
| Class | Typical meaning |
|---|---|
| Critical | Could harm the user or breaks a legal or safety requirement (such as a broken needle in a garment, or a sharp object) |
| Major | Likely to cause the garment to fail in use or to be returned; clearly visible on a main part |
| Minor | Unlikely to affect use or sale but is a departure from standard |
Buyers define these classes in their own manuals, with examples. Always inspect to the buyer's definitions.
What AQL does and does not tell you#
- AQL is a sampling decision rule, not a quality target. Passing an AQL 2.5 inspection does not mean the lot has 2.5% or fewer defectives; it means the sample result was within the accept number.
- Small samples have risk. A good lot can fail and a poor lot can pass by chance. The tables balance this risk; they do not remove it.
- AQL does not fix quality. Quality comes from process control on the line: inline inspection, training and root cause work.
Counting defects or defectives#
Some manuals count defective units (a garment with several defects counts once in its worst class); others count defects. Check the manual. The difference changes the result.
Common mistakes#
- Choosing samples only from easy-to-reach cartons. Draw samples from across the lot at random.
- Inspecting before the lot is complete and calling it a final inspection.
- Mixing up lot size and order quantity when shipping in several lots.
- Using the wrong table row after rounding the lot size.
- Reworking the sample and passing the lot without addressing the rest.
See final inspection for how an AQL inspection is carried out, and use the AQL inspection sheet to record results.
Related guides#
- DHU: measuring quality on the line before final inspection.
- Sewing defects: the defects most often found in AQL samples.
- Right first time: first-pass quality, which decides how many lots pass.
- Process flow chart and layout of garments finishing: the finishing checks before the lot is offered.