Acceptance Quality Limit (AQL) for Product Inspection
ISO 2859-1 defines acceptable quality level (AQL) as the “quality level that is the worst tolerable.” What this means is that, AQL, expressed as a percentage or ratio of the number of defects compared to the total quantity, measures how many defective components are considered acceptable during random sampling quality inspections. AQL refers to the maximum number of defective units, beyond which the lots containing the defects are rejected.
AQL Parameters
These parameters are used in AQL tables to determine the acceptable level of production run.
Lot size
Lot size refers to the quantity of a product manufactured in a single production run. That is, the total quantity of a product ordered for manufacturing. If different products are ordered, each product is considered as a separate lot. If you ordered only one product, the lot size is the total batch quantity. The quantity of each product is the lot size.
Inspection Level
Inspection level refers to the number of samples to check. They are categorized into three (3), namely, level I, II & III. Different inspection levels commands different numbers of samples to inspect. Level I entail checking fewer samples, level II are in-between level I & III and are acceptable for most importers, while Level III involves tightening up and checking more samples.
The AQL limits
The AQL limits provide an upper limit to defective products. It is wise to set a lower AQL for both major and minor defects if the customer accepts very few defects.
AQL Tables
These tables are a set of statistical tools (charts) for product inspection that allow users to easily determine the number of samples that are necessary for testing and the number of allowable defective units based on a given AQL.
How Acceptable Quality Level (AQL) Works
Manufactured parts are often randomly tested according to some defined sampling plans. If the number of defective items falls below the predetermined amount, such a product will be said to meet the acceptable quality level (AQL). If the acceptable quality level (AQL) is not reached for a particular inspection exercise, a review of the various parameters in the production process is conducted to determine the possible and root cause of the defects for an appropriate corrective action.
For instance, say we are to conduct an AQL of 2% on a specific manufacturing batch, meaning that no more than 2% of the batch should be found with defects. If the batch is made up of say, 1,000 products, only 20 products can be defective. If more than 20 (e.g. 20.5) products are found defective, the entire batch is scrapped.
It should be noted that AQL of products are not fixed, in that, they depend on the type of industry. AQL of medical equipment manufacturers, metallurgical, food production, electronics, pharmaceutical, automobiles industries etc. are all different. They depend on the nature of the products and their effect on their consumers. Hence, pharmaceutical products are more likely to have more stringent AQL than say, metallurgical products, because defective pharmaceutical products can result in severe health risks to the vast majority.
Some factors being considered while setting up an acceptable quality limits could include safety, business and financial implication of a products and their associated defects.
Categories of AQL Defects
Defects are simply a fault, or anything that makes a product or service not useful. Defects can be categorized into three:
Critical defects
Defects, when accepted could harm users. These defects cannot be accepted because of the danger they pose. They are designated as 0% AQL.
Major defects
These too are not acceptable by the end-users, as they are likely to result in failure when in use. Hence, they are termed ‘major’ with designated of AQL of 2.5%.
Minor defects
These products differs from specified standard but are likely not going to reduce the usability of the products , as they can still be ‘managed’ by the users. The AQL for minor defects is 4%.
About the Author
Adebayo is a thought leader in continuous process improvement and manufacturing excellence. He is a Certified Six Sigma Master Black Belt (CSSMBB), Digital Manufacturing Professional and ISO Management Systems Lead Auditor (ISO 9001, 45001 & ISO 22000) with strong experience leading various continuous improvement initiative in top manufacturing organizations.
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