During the process of product inspection, the Acceptance Quality Limit (AQL) has become a widely adopted industry standard for conducting sampling. Formerly known as Acceptable Quality Level, AQL was initially formulated by Harold F. Dodge several decades ago and has since undergone refinement by him and other statistical experts.
In today’s landscape, AQL stands as a recognized and effective approach for random sampling during product inspection, taking into account calculated risks for both customers and suppliers. It provides a quantifiable benchmark, offering both buyers and suppliers clear guidance on the acceptable number of defective products within a single inspection, in accordance with AQL sampling guidelines.
Understanding Defects in Product Inspection
Before delving into AQL, a fundamental concept to grasp is the notion of defects. Given the realities of mass production, variations in product defects are inevitable and can significantly impact the economic interests of buyers.
Primarily conducted as a final random inspection, product inspection serves as an objective and measurable assessment of the quality of a batch of products set for delivery. By obtaining inspection results, buyers gain insights into potential defects within the product batch, aiding in decision-making regarding goods disposition.
Interpretation of defects varies among different buyers and often involves classifying defects as critical, major, or minor. Typically, buyers outline their expectations for defect definitions within their inspection protocols or manuals for inspector guidance.
Broadly speaking, these three defect types are understood as follows:
1. Critical defects: Defects that violate mandatory regulations and/or pose safety risks to consumers or end-users.
2. Major defects: Defects that lead to product malfunction, significantly reducing the product’s usability or marketability.
3. Minor defects: Defects that deviate from quality standards but do not substantially impair the product’s usability or marketability.
For instance, a needle discovered in a pair of children’s trousers would be classified as a critical defect, a malfunctioning battery in a flashlight as a major defect, and a minor scratch on a wardrobe as a minor defect.
Understanding General and Special AQL Sampling Levels
The AQL framework encompasses two main sampling levels: General Sampling Level and Special Sampling Level. The latter is typically reserved for specialized purposes such as destructive testing.
Within the General Sampling Level, three sub-levels exist: GI, GII, and GIII, corresponding to ‘Reduced,’ ‘Normal,’ and ‘Tightened’ sampling levels, respectively. The sampling size-to-lot size ratio escalates from GI to GIII. For most mass consumer products, GI and GII are standard for routine inspection.
Deciphering AQL Level Setting
Buyers commonly exhibit varying degrees of stringency and quality expectations, leading to diverse AQL level settings.
In standard inspections, AQL levels range from 0.065 to 6.5, with higher AQL values indicating greater leniency in inspection criteria.
For general consumer product inspections, an AQL level of 2.5 is typical, signifying zero tolerance for critical defects, 2.5 for major defects, and 4 for minor defects.
Embarking on AQL Implementation for Product Inspection
Once inspection and AQL levels are established, AQL can be effectively applied to the inspection process. In the initial AQL table, assuming a lot size of 1000 and a GII inspection level, the letter code J is derived.
Referring to the second AQL table and utilizing the letter code J, one can determine that the sampling size for the batch is 80 units. Assuming an AQL level of 2.5, the acceptance number becomes 5, while the rejection number is 6. In other words, the AQL dictates that the batch should be accepted if five or fewer defects are identified during inspection, but should be rejected if six or more defects are found.
Nevertheless, certain scenarios may lead to instances where the letter code and AQL level result in an empty cell. For instance, if the letter code is N and the AQL level is 4, acceptance and rejection numbers become 21 and 22, respectively.
Product inspection serves as a valuable tool in comprehending the quality of goods before shipment. Effective utilization of AQL provides insightful, objective measurements of quality, empowering buyers to make informed decisions about their procurement orders. For buyers seeking assistance in developing their inspection programs and requirements, we are well-equipped to lend our industry expertise and experience.