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AQL Inspection Manual

AQL (Acceptable Quality Level)

When a consumer designates some specific value of AQL for a certain defect or group of defects, he indicates to the supplier that his (the consumer's) acceptance sampling plan will accept the great majority of the lots or batches that the supplier submits, provided the process average level of percent defective (or defects per hundred units) in these lots or batches be no greater than the designated value of AQL. Thus, the AQL is a designated value of percent defective (of defects per hundred units) that the consumer indicates will be accepted most of the time by the acceptance sampling procedure used.

The sampling plans in the MIL-STD-105E, ANSI/ASQ Z1.4 and ISO 2859-1 and standards are so arranged that the probability of acceptance at the designated AQL value depends upon the sample size, being generally higher for large samples than for small ones, for a given AQL. The AQL alone does not describe the protection to the consumer for individual lots or batches but more directly relates to what might be expected from a series of lots or batches. Altogether there are 26 specific values of AQL. The AQL Inspector's Rule covers the range from 0.065 to 15 which includes the vast majority of sample plans used. Notes are available online for using the AQL Inspector's Rule for AQL values below 0.065.

For inspection under MIL-STD-105E, ANSI/ASQ Z1.4 or ISO 2859-1, the AQL will be specified in the contract or specification. Different AQL's may be designated for groups of defects consider collectively, or for individual defects. A common practice is to designate different value of AQL for major, minor or total defects. Due to the difference in the specified AQL values, it is possible that one sample size be indicated in the sampling procedures for major defects, and another sample size for minor defects or total defects. Whenever two or more sample sizes are indicated by Tables (for a given sample size Code Letter but for different values of AQL), the correct procedure is to select the largest indicated sample at random from the lot, then select the smaller sample sizes at random from the larger sample. An AQL for a group of defects may be designated in addition to AQLs for individual defects, or subgroups, within that group. AQL values of 10 or less may be expressed either in percent defective or in defects per hundred units: those over 10 shall be expressed in defects per hundred units only.