The acceptance number is the maximum number of defects or defective units in the sample that will permit acceptance lot or batch.

Acceptance Sampling is used to decide whether to accept or reject a production lot. After selecting a sampling from a production lot, we inspect the units, we count how many defects are found among this sampling and we use the result (number of defects found versus maximum number of defects allowed) to decide whether or not to accept or reject the lot.

The AQL has two different definitions due to standard changes.

1. Acceptable Quality Level / MIL-STD-105E / ISO 2859-1 (1999)

The acceptable level (AQL) is defined as the maximum percent defective (or the maximum number of defects per hundred units) that, for purpose of sampling inspection, can be considered satisfactory as a process average. The sampling plans most frequently used by the department of Defense are based on the AQL.

2. Acceptance Quality Limit / ANSI/ASQC Z1.4-2003

The AQL is the quality level that is the worst tolerable process average when a continuing series of lots is submitted for acceptance sampling.

The following note on the meaning of AQL was introduced with the ANSI/ASQ Z1.4-2003 revision.

The concept of AQL only applies when an acceptance sampling scheme with rules for switching between normal, tightened and reduced inspection and discontinuance of sampling inspection is used. These rules are designed to encourage suppliers to have process averages consistently better than the AQL. If suppliers fail to do so, there is a high probability of being switched from normal inspection to tightened inspection where lot acceptance becomes more difficult. Once on tightened inspection, unless corrective action is taken to improve product quality, it is very likely that the rule requiring discontinuance of sampling inspection will be invoked.

Although individual lots with quality as bad as the AQL can be accepted with fairly high probability, the designation of an AQL does not suggest that this is necessarily a desirable quality level. The AQL is a parameter of the sampling scheme and should not be confused with a process average which describes the operating level of a manufacturing process. It is expected that the product quality level will be less than the AQL to avoid excessive non accepted lots.

The AQL values are defined as percent nonconforming or defects or nonconformities per hundred units.