Almost every inspection report mentions AQL, and many buyers nod along without knowing what it means. That matters, because the number printed on your report decides whether a shipment of 10,000 units is accepted or rejected. A short explanation of how AQL inspection works will help you set better terms with your suppliers and read reports with more confidence.
AQL stands for Acceptable Quality Limit. It is the maximum number of defective units, expressed as a percentage, that you are prepared to accept in a lot. Checking every unit is slow and expensive, so inspectors pull a random sample and use the number of defects found to decide on the whole lot. The method is statistical, which means it accepts some risk. A lot with a few defects may pass, and a lot with slightly more may fail, even though no one has looked at every piece.
Three choices drive the result. The first is the inspection level. General level II is the usual default. Level I uses a smaller sample and is cheaper, and level III uses a larger one and is stricter. The second is the AQL value for each defect class. The third is the lot size, which determines the sample size through a standard table, usually ISO 2859-1.
Defects are sorted into classes. A critical defect is one that makes the product unsafe or breaks a legal requirement, such as a sharp edge on a child’s toy or an exposed live wire. Most buyers allow none. A major defect makes the product unfit for its purpose or visibly poor, like a zipper that does not close or a cracked housing. A minor defect is a small departure that does not affect function, such as a loose thread. Common AQL settings are 0 for critical, 2.5 for major, and 4.0 for minor.
Here is a worked example. You order 5,000 units. At general level II the table gives a sample size of 200. At AQL 2.5 for major defects, the lot is accepted if the inspector finds 10 or fewer major defects in those 200 units, and rejected at 11 or more. At AQL 4.0 for minor defects the limit is 14. For critical defects the limit is 0, so a single one fails the lot. These figures come from the standard table, and your inspector will confirm them for your lot size.
Notice what the numbers mean in practice. Ten major defects in 200 units is a rate of five percent, which is double the nominal 2.5. Buyers are sometimes surprised that a lot with that many flaws can pass. The sampling plan is built so that lots with a true defect rate near the AQL are usually accepted, and lots with a much higher rate are usually rejected. If a five percent major defect rate hurts your business, choose a tighter AQL, such as 1.5 or 1.0, and accept that a larger sample and more rejections follow.
Several things sit outside the tables. AQL applies to each lot, and a lot means one product, one colour, or one size, depending on how you define it. An order with eight colours is eight lots. Packaging and labelling defects, measurements, and functional tests are usually checked on the same sample, but some buyers set separate, smaller samples for tests that take a long time. Safety-related features often get a zero-tolerance rule outside AQL altogether.
Be wary of a factory that proposes its own sampling plan. Smaller samples and looser limits are easy to suggest, and they favour the factory. Put the inspection level, sample size, and AQL values in your purchase order or quality agreement so there is nothing to negotiate on the day. InspectionService.com and other providers of third-party pre shipment inspection services will work to the plan you specify and can advise on suitable levels for your product category.
A pass on AQL tells you the lot meets the agreed standard. It does not guarantee that every unit is perfect. If the cost of a single defect is very high, as with medical items or safety parts, add 100 percent checking on the critical feature. For ordinary consumer goods, a sensible AQL plan applied consistently, with the results tracked order by order, gives you a clear and fair way to hold suppliers to a standard.
