AQL Sampling Plans, Made Simple: The Tables You Actually Need

May 2026 · 6 min read · Quality Control

Most inspection contracts in China quote "AQL 2.5, level II". Behind that short phrase sits a set of tables (ISO 2859-1 / ANSI Z1.4) that determine how many units get checked and what results mean pass or fail. Here is how to read them without a statistics degree.

The three inputs

• Batch size: the total number of units in the lot being inspected.
• Inspection level: I (lighter), II (default), III (heavier). Most buyers use level II.
• AQL value: the acceptable quality limit for each defect class (e.g. 2.5 major, 4.0 minor).

The core table at level II

Read a row like this: for a 10,000-unit batch at AQL 2.5, the inspector checks 200 units. If 7 or fewer have major defects, the lot passes. If 8 or more, it fails.

What inspectors actually do

• Draw the sample randomly across the whole lot — not from the top of one pallet.
• Classify each defect as critical, major or minor using the agreed standard.
• Count defects per class and apply the accept/reject numbers independently per class.
• One critical defect anywhere fails the whole lot, regardless of the numbers.

Four mistakes to avoid

• Using one AQL for everything: criticals should be 0, majors usually 1.0–2.5, minors 4.0.
• Forgetting that "Accept" is defects, not defective units — one unit can carry two major defects.
• Letting the supplier's own staff select which cartons to open.
• Inspecting without the agreed product spec on hand — without a spec, nothing can be judged.

The honest limit

AQL sampling is a risk-control tool, not a guarantee. A passing lot can still contain defects — that is the statistical trade-off every buyer accepts. What matters is that the same rules are used by both sides and written into the contract before problems appear.