AQL table: how to read an ISO 2859-1 sampling plan

The three steps

1. Lot size and inspection level give the code letter. ISO 2859-1 has three general inspection levels (I, II, III) and four special levels (S-1 to S-4). General level II is the default for most pre-shipment inspections. Level III takes a larger sample; the special levels take small samples for long or destructive tests.
2. The code letter gives the sample size. Each letter matches one sample size: H is 50 pieces, L is 200 pieces, M is 315 pieces.
3. Your AQL gives the accept and reject numbers. In the column of your AQL, Ac is the maximum number of defects that still accepts the lot, and Re is the number from which the lot is rejected.

Table for lots of 281 to 35,000 pieces

General inspection level II, single sampling, normal inspection.

Source: ISO 2859-1, Table 1 (sample size code letters) and Table 2-A (single sampling plans for normal inspection). For other lot sizes, levels or AQL values, use the standard: in some cells an arrow sends you to the neighbouring plan.

A worked example

A buyer orders 5,000 machined parts with these limits: critical defects 0, major defects AQL 2.5, minor defects AQL 4.0. The table gives code letter L and a sample of 200 pieces. The inspector counts the defects in the sample by class:

With 11 major defects, or a single critical defect, the lot would be outside the limits. ISO 2859-1 has no AQL column for zero: "0 critical defects" is a rule the buyer sets in the inspection protocol, and any critical defect found puts the lot outside it.

Critical, major, minor: who decides?

The buyer does. The classification belongs in the inspection protocol, written before the inspection. For mechanical and electrical parts, a typical split is:

• Critical: a defect that can make the product unsafe, such as a crack in a load-bearing part or a missing insulation component.
• Major: a defect that affects function or assembly, such as a critical dimension out of tolerance or a thread that does not engage.
• Minor: a defect that does not affect use, such as a small cosmetic mark.

What AQL does not tell you

• AQL 2.5 does not guarantee that the lot contains less than 2.5% defective parts. It is the quality level that the sampling plan accepts most of the time; a lot with more defects can still pass by chance.
• Dimensional, functional and destructive tests usually use their own, smaller samples, set in the protocol, because they take longer per piece.
• When a single defective part is unacceptable for your customer, sampling is not enough. 100% inspection and sorting checks every unit.

How MSC applies AQL

Each inspection follows a written protocol agreed with you: your own, our standard checklist, or a protocol we draft for your approval. The inspector records the defects found by class, the measured values and photos, and the report reaches you within 24 hours. The decision to release the lot stays with your quality manager. Inspections cost $240 per inspector-day, all inclusive anywhere in mainland China.