AQLs

Acceptable Quality Level (AQL) is a statistical threshold used in acceptance sampling to describe the worst tolerable defect rate in a batch or lot that a supplier is willing to accept. It does not guarantee the lot is defect-free; instead it defines the level at which the sampling plan has a stated probability of accepting defects. AQL is often described as the maximum average defect rate that is acceptable for a production process, and it is related to consumer risk and producer risk.

AQL is applied via sampling plans that specify sample size n and acceptance number c. If the number of defective items in the sample is ≤ c, the lot is accepted; otherwise rejected. AQL values are specified in standards such as ISO 2859-1 and ANSI/ASQ Z1.4. Common categorization uses three defect levels—critical, major, and minor—each with its own AQL. In practice, products with critical defects may be rejected outright; major and minor defects have definable AQLs.

Limitations: AQL is a probabilistic tool and depends on factors such as lot size, inspection method, and the distribution of defects. It is not a guarantee of quality and may allow non-conforming items to pass if they occur outside the sampled items. It is less suitable for high-stakes safety items and should be used in conjunction with other quality management practices to monitor and improve production processes. Variations exist across industries, but the core concept remains a standardized basis for lot-by-lot decision-making.