Acceptable Quality Level (AQL)
AQL：Acceptable Quality Level
MA: Major Defects
MI: Minor Defects
N: Lot size / batch size
n: sample size
The acceptable quality level (AQL) is a measure applied to products and defined in ISO 2859-1 as the “quality level that is the worst tolerable.” The AQL tells you how many defective components are considered acceptable during random sampling quality inspections. It is usually expressed as a percentage or ratio of the number of defects compared to the total quantity.
The acceptable quality level (AQL) is the worst quality level that is tolerable for a product.
The AQL differs from product to product. Products that might cause more of a health risk will have a lower AQL.
Batches of products that do not meet the AQL, typically based on a percentage measurement, are rejected when tested during pre-shipment inspections.
The current international inspection standard is ANSI/ASQ Z1.4/MIK-STD-105E/
One key thing to understand before talking about AQL is defect. For mass production, it is inevitable to find products with different levels of defects, which can directly impact the economic benefits to the buyers.
Inspection, mainly conducted as a final random inspection, provides an objective and measurable quality assessment of the batch of products that the buyers are going to receive. With the results from the inspection, buyers will be able to have a picture of the possible defective problems with the batch of products, and thereby make a decision concerning the goods arrangement.
Different buyers may have different interpretation of defects, which are further classified as critical defects, major defects, and minor defects. Usually, buyers specify their expectations in the defect definition in their inspection protocols or manuals for inspectors’ execution.
Critical defects - a defect likely to result in a hazardous or unsafe condition for an individual using the product or fails to meet mandatory regulations.
Major defects - a defect that is likely to result in failure, reduce the usability & salability and can easily be detected by the customer.
Minor defects - a defect that does not reduce the usability of the product, can be noticed only after careful examination, and may reduce the salability. A workmanship defect beyond the defined quality standard.
Goods in a sample are inspected at random, and if the number of defective items is below the predetermined amount, that product is said to meet the acceptable quality level (AQL). If the acceptable quality level (AQL) is not reached for a particular sampling of goods, manufacturers will review the various parameters in the production process to determine the areas causing the defects.
Determine inspection level
The AQL guideline consists of two sampling levels: the General Sampling Level and Special Sampling Level. The latter, as the name suggests, is adopted mainly for special purposes, such as destructive checking.
There are three sub-levels under General Sampling Level, namely GI, GII and GIII, which represent 'Reduced', 'Normal' and 'Tightened' sampling respectively. The ratio of sampling size to lot size increases from GI to GIII level. In most cases of mass consumer products, GI and GII are used for normal inspection.
Set Acceptable Quality Level
The acceptable quality level is the number of allowable defects in a batch. The AQL is agreed upon between the buyer and the supplier based on the importance of the product. Under normal inspection, AQL levels range from 0.065 to 6.5. The larger the AQL level, the more lenient the inspection. For general consumer products inspection, AQL level is usually set at MA:2.5, MI:4.0 which implies a zero tolerance for critical defect, AQL 2.5 for major defects, and AQL 4.0 for minor defects.
Select Sampling Method
A few sampling methods can be used to achieve quality assurance based on the level of sophistication and rigor required. The key methods are as follows:
-Single sampling is the most common method. It only requires one sample of size n and the number of defaults c. Hence, it is also called (n,c)-sampling. If in the n samples, defectives are greater than defaults c, the entire batch is rejected.
-Double sampling is an extension of the single sampling method. In such a case, if the first sample is indecisive, then a second sample is taken to make the decision.
-Sequential sampling is an elaborate method where each item from the sample is tested, and a decision to accept, reject, or continue testing is made after every item is tested.
-The other methods are multiple sampling and skip lot sampling.
The AQL chart comprises two parts – the first one is the inspection level chart. It is used to choose the inspection level based on the batch size and the level of scrutiny that is required. For example, assume a batch of 5,000 shirts needs to be inspected, and the level of inspection is set at GII. The level of inspection, according to the prescribed table, is at the intersection of the batch size row and inspection level column. As shown in the table below, the code letter is "L"
AQL Charts - Sample Size Code Letters
The next step in the process is to determine the appropriate sample size, pick the AQL, and make a decision based on thresholds given by the chart. The thresholds are determined similarly at the intersection of inspection level row and AQL column.
For example, Assuming the AQL to be MA:2.5 and MI:4.0, we get that the the sample size of the code letter L is 200(200 shirts were randomly selected for inspection out of 5,000) and the number corresponding to AC is MA:10 and MI:14. If there are 11 (or more) major defects or 15 (or more) minor defects were found during the inspection, the buyer should reject the entire batch.
AQL Charts - Single Sampling Plans