Maintaining the quality of products or services is essential for all industries and businesses. Some quality control concepts help the business to produce quality products. AQL is one of the important concepts.
This article gives a clear picture of the Acceptable Quality Level. In addition, the article informs you about categories of defects in AQL and helps you understand the AQL table and sampling process.
What is Acceptable Quality Level?
Acceptable Quality Level is purely a concept of quality control. In detail, it is the set number of defective products that are acceptable for a selected sample size. It is defined in ISO 2859-1 as a “quality level that is the worst tolerable.”
While doing a random sampling quality inspection, AQL helps you to know how many defective products are considered acceptable.
AQL is always indicated as a ratio of the number of defects compared to the total lot or quantity.
What are the Categories of defects in Acceptable Quality Level?
Generally, defects are categorized as follows.
1. Critical defects: In this category, the accepted defects could harm the users. This kind of defect is not acceptable. So it is defined by 0%AQL.
2. Major defects: In this category, defects are not acceptable by the end-users, as it is likely to fail. The AQL for major defects is 2.5%
3. Minor defects: In this category, defects do not reduce the product’s usability for its intended purpose. Some end-users will still buy such products. The AQL for minor defects is 4%.
As long as the production level yields to defective pieces within the AQL range, it means that manufacturing is smooth.
If the number of rejections is higher than the set AQL, then the whole batch will be rejected, and you come to know that the manufacturing processes will have to be reviewed and the defect causing areas to have to be rectified. So it is an essential tool in Six Sigma quality control.
The acceptable quality level depends on the type of products and differs from product to product and from industry to industry.
Products used in building airplanes will have very little AQL as compared to a company that manufactures toys.
Acceptable quality level example
For example, a toy manufacturing company sets an AQL of 1% on a lot. That means out of 1000 toys in one lot, only 10 toys can be defective. If the faulty toys are more than ten, the entire batch should be scraped.
A manufacturing company can set an AQL of 1% on a lot, but a Pharmaceutical company could set an AQL of 0.001% or 0% on a lot because defective products can be risky for health.
Hence, the AQL must be calculated based on the appropriate size, sample, and industry.
When the sampling is done, the products are accepted if the statistical values lie below the expected AQL.
We know that AQL means the worst quality level. However, apart from AQL, two more quality levels are there in the quality control department.
RQL(Rejectable Quality Level): This is known as unsatisfactory quality level or lot tolerance percent defect (LTPD).
IQL(Indifference Quality Level): This level lies between RQL and AQL. That will be different for different companies.
Acceptable Quality Level is one of the crucial parameters in the sampling process. The other two are Inspection level and sampling method.
Inspection levels are of two types: General inspection level and special inspection level.
How many methods of sampling are there?
Generally, five methods of sampling are there. They are single sampling, double sampling, sequential sampling, multiple sampling, and skip lot sampling.
- Single sampling: In this sampling method, only one sample will be taken for inspection out of n-number of samples. If any fault is identified in that sample, then the whole batch will be rejected.
- Double sampling: This method is the continuation of 1st method. That means when the first sample fails, then one more sample is to be taken for inspection.
- Sequential sampling: In this method, every item from the sample will be tested. It is a precise method.
- Multiple sampling: This method is an add-on to double sampling. In this method, a random sample size of n will be taken from a large lot of size N.
- Skip lot sampling: In this method fraction of the lots will be inspected.
Acceptable Quality Level ( AQL) Table
What is an AQL table?
AQL table is an industrial standard statistical table that helps determine the number of samples to be picked and inspected in a batch.
To understand the acceptable quality level table following parameters are to be kept in mind.
- Lot Size: Lot size is the number of products that you ordered. Suppose you order different products, each product is to be considered as a separate lot.
- General inspection level: It is a typical inspection level
- Special inspection level: It is a little bit different from the general inspection level. In this inspection level, the sample size will be small compared to the general inspection level, and some special inspection to be conducted for the sample
- Sample size code letters: It represent the different sample size
- Sample sizes: It is the number of units that you take randomly for inspection
- Acceptable quality level: It represents your tolerance for defects in a lot. Usually sets in terms of percentage
- Acceptance point: It is the maximum number of defects acceptable in a given sample size.
- Rejection point: It is the threshold point for rejecting an order based on the defects in a given sample.
Get more definitions about Acceptable quality levels and other ERP-related terms here.