Overview of AQL Inspection Standards
I. Basic Definition of AQL
Acceptable Quality Level (AQL) is a crucial metric in the field of product inspection. It defines the maximum allowable defect limit in the inspection of products within a specific sample size. Once this limit is exceeded, the corresponding batch of products will be rejected. It is usually presented as a percentage or ratio of the number of defects to the total number of products.
II. Classification Settings of AQL
During the production quality control process, defects are generally classified into three categories:
1. Critical Defects: Based on experience and judgment, such defects are highly likely to cause the user to be in a dangerous or injured situation, or result in the product being legally seized, or as specified in the contract. The criteria for defining it include constituting a safety hazard to the expected user, triggering product recalls, and causing brand - damaging defects. For example, the presence of dangerous sharp points or edges on the packaging, the lack of the local language of the destination, poor assembly, damage of the power switch, the connector not covered with an insulating tube, and the detection of needles.
2. Major Defects: Apart from critical defects, this type of defect may lead to product failure or significantly reduce the usability of the product for its intended purpose, or as specified in the contract. The judgment criteria include affecting the performance or function of the product, changing the product specifications, and causing the end customer not to buy or return the product. For example, skipped stitches in the product, the product cannot be assembled smoothly, incorrect type or rating of the bulb, and failure to reach the rated voltage.
3. Minor Defects: This type of defect basically does not reduce the usability of the product for its intended purpose, or has a slight deviation from the established standards with little impact on the effective use of the product, or as specified in the contract. The criteria for defining it are that it does not affect the function or use of the product, is barely noticeable at a normal viewing distance, and is unlikely to cause a return or make the product unsalable. For example, dirty stains, untrimmed thread ends, glue marks, and poor paint quality of the product.
The AQL levels are different for different types of defects. For products that may pose a greater health risk, the AQL value for critical defects is usually set at 0%; the AQL value for major defects is generally set at 2.5% (these products are usually not acceptable to the end user); the AQL value for minor defects is generally set at 4.0% (there are some deviations from the specifications, but most users can accept them).
III. Relevant Parameters of AQL
1. Lot Size: According to the ISO 2859 standard, the lot size refers to the number of items in a batch of products. If multiple products are ordered, each product is regarded as a separate batch, and its quantity is the lot size; if only one product is ordered, the total number of this product is the lot size.
2. Inspection Level: It represents the relative amount of inspection. Generally, three common inspection levels are provided, namely Ⅰ, II, and Ⅲ. In the absence of special instructions, level II is usually used. Level Ⅰ is suitable for situations where less discrimination is required, and level Ⅲ is used for situations where higher discrimination is required. In addition, there are four special inspection levels, S - 1, S - 2, S - 3, and S - 4, which are used in cases where a relatively small sample size is required and a larger sampling risk can be tolerated.
3. AQL Limits: When consecutive batches of products are submitted for acceptance sampling, the AQL limit reflects the worst tolerable process average quality level. The AQL limit should be set according to the market situation. If the customer has a low tolerance for defects, a lower AQL value can be set for major and minor defects.
IV. Use of AQL Tables
The AQL table is a common statistical tool for product inspectors and is one of the industry standards. It mainly includes two tables:
1. The first table is the sample size code letter table. For example, when the lot size is between 3201 and 10000 pieces and the inspection level is II, the corresponding code letter is "L".
2. The second table is the single sampling plan table for different inspection levels (taking the normal severity of level II inspection as an example). When the code letter is "L", 200 pieces need to be randomly selected from the total lot. Assuming that the AQL for major defects is set at 2.5% and the AQL for minor defects is set at 4.0%, then the products are acceptable when no more than 10 products with major defects and no more than 14 products with minor defects are found; otherwise, the products will be rejected.
Click here to download the AQL table