In brief
AQL (acceptance quality limit) is the quality level used in sampling inspection to decide whether a lot is accepted or rejected.
Contents
What is AQL?
The AQL value indicates what proportion of nonconforming units, as a process average, is still considered acceptable. It is at the core of sampling inspection to ISO 2859-1, which describes procedures for inspection by attributes: rather than checking every part, a sample drawn according to fixed rules is inspected. An important distinction: an AQL says nothing about individual parts and does not guarantee a defect-free lot.
The lot size and inspection level give a code letter for the sample size. The general inspection levels I, II and III are common, along with special inspection levels for costly or destructive tests. For each AQL value, the table gives an acceptance number and a rejection number: if no more defects are found than the acceptance number, the lot is accepted; from the rejection number onwards it is rejected. Depending on supplier history, inspection switches between normal, tightened and reduced.
In procurement, AQL is applied during incoming inspection and pre-shipment inspection. Separate AQL values are usually agreed for critical, major and minor defects.
In practice: maintenance and procurement
AQL values, inspection level and the classification of defects should be set out in writing in the purchase order or quality assurance agreement before inspection takes place. For safety-relevant spare parts, a sample is often not sufficient; here, 100 % inspection or inspection backed by an inspection document may be more appropriate.
Synonyms and abbreviations: acceptance quality limit · acceptable quality level · AQL sampling · ISO 2859-1 · AQL inspection
Frequently asked questions
What does AQL 2.5 mean?
AQL 2.5 means that a process average of 2.5 per cent nonconforming units is considered just acceptable. It does not follow that every lot may contain exactly that many defects. How many defects are permissible in the sample only follows from the lot size, the inspection level and the corresponding table in the standard.
How do you determine the sample size according to AQL?
First, the code letter is determined from the lot size and the chosen inspection level. In the sampling plan of ISO 2859-1, this code letter leads to the sample size. Together with the agreed AQL value, the acceptance and rejection numbers are read off there. The usual starting point is general inspection level II with normal inspection.
What is the difference between critical, major and minor defects?
Critical defects can lead to hazardous conditions or prevent functioning altogether; often no defect at all is tolerated for them. Major defects significantly impair fitness for use, minor defects hardly at all, for example small cosmetic flaws. As a rule, a separate AQL value is set for each defect class.
Does an AQL inspection guarantee defect-free goods?
No. Sampling inspection works with probabilities: an accepted lot may still contain defective parts, and a good lot may occasionally be rejected. AQL inspection is an economical way of limiting risk, but it does not replace 100 % inspection where this is technically or safety-wise necessary.