AQL sampling explained

01How a plan is read

1. Lot size and inspection level (usually general level II) give a sample size code letter.
2. The code letter and the AQL give the sample size and the acceptance (Ac) and rejection (Re) numbers.
3. If the number of nonconforming units in the sample is at or below Ac, the lot is accepted; at or above Re, it is rejected.

Try it with the AQL sampling calculator.

02Defect classes

Defects are classified as critical, major or minor, each with its own AQL. Common practice in industrial PSI is zero acceptance for critical defects, AQL 1.0 to 2.5 for major and AQL 2.5 to 4.0 for minor, but values must be agreed in the purchase order.

03What AQL is and is not

The AQL is the quality level that, with this plan, is accepted most of the time. It is not a guarantee that accepted lots contain fewer defects than the AQL, and it is not a permitted defect rate. Lots with worse quality may still be accepted with some probability, described by the plan's operating characteristic curve.

04Inspection levels and switching

Level I gives smaller samples and less discrimination, level III larger samples. Special levels S-1 to S-4 are for expensive or destructive tests. For continuing supply, ISO 2859-1 defines switching rules between normal, tightened and reduced inspection based on recent lot history.

05Practical advice

• Define defects with drawings, limit samples and photos before inspection.
• Draw samples randomly from the whole lot, not only from accessible cartons.
• Use 100% inspection for critical characteristics where possible.
• Do not use AQL as a substitute for process control.