Also called: Acceptable Quality Level
An AQL sets the highest percentage of defective units (or defects per hundred units) that is still acceptable on average when a production lot is checked by sampling rather than unit by unit. The sampling plans themselves come from ISO 2859-1 (the equivalent of the older MIL-STD-105 / ANSI Z1.4 tables): lot size and inspection level give the sample size, and the AQL gives the maximum number of defective units allowed in that sample.
Orders usually state separate AQLs for critical, major and minor defects; in apparel, values such as 2.5 for major and 4.0 for minor defects are common, while critical defects are often set to zero. Put the AQL, the inspection level and the defect definitions in the specification so every supplier quotes against the same acceptance rule.
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