What is an Acceptable Quality Level (AQL)? – Defect categories

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definition of Acceptable Quality Level (AQL)

AQL is a term used in quality control, and it has different meanings depending on the context.

This blog post will provide an overview of the Acceptable Quality Level, including its definition, categories of defects, sampling processes, and the table.

What is an Acceptable Quality Level?

In quality control, AQL refers to the maximum allowable number of defects per unit that a lot can have and still be considered acceptable for delivery to the customer. The adequate quality level is calculated with the size of each lot and defined as a percentage or number.

It is defined in ISO 2859-1 as a “quality level that is the worst tolerable.”

It can also refer to the acceptable quality limit, a method used for sampling inspection that determines whether a sample of some product meets specific requirements.

Categories of Defects in the AQL

There are three main categories of defects that can be found in a lot:

  • Major defect
  • Minor defect
  • Critical defect
Categories of Acceptable Quality Level

Major defect

A major defect is a flaw in the product that renders it unsuitable for its intended use. This defect can cause the product to be unsafe, unusable, or aesthetically displeasing.

The typical AQL for major defects is 2.5%

Minor defect

A minor defect is a flaw in the product that does not meet its standards but does not render it unusable, unsafe, or aesthetically displeasing.

The typical AQL for minor defects is 4%

Critical defect

A critical defect is a flaw in the product that meets none of the above criteria and affects at least one unit. Critical defects can result in unsafe, unusable, or aesthetically displeasing products.

Manufacturing is smooth if the production level yields defective pieces within the AQL range.

If the number of rejections is higher than the set level, 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.

Six sigma alnd aql

The acceptable quality level depends on the type of products and differs from product to product and industry to industry.

Products used in building airplanes will have very little AQL compared to those that manufacture toys.

Acceptable quality level example

Acceptable quality levels can be expressed as a percentage or a number. For example, one unit’s typically permissible quality limit ranges from 0 to 100%.

For example, The AQL of 20% means that if there are 500 units in the lot and 50 of them have defects. Sampling process Acceptable Quality Limit tables provide the minimum sample sizes needed to detect specific percentages of defects at some confidence level. It works by taking a sample set from each lot and determining whether it meets requirements based on its characteristics. For example, let’s say you have 1000 widgets in your warehouse.

Sampling process

When using Acceptable Quality Level tables, two main sampling processes are used.

Acceptance sampling

Acceptance sampling is a type of sampling where a certain number of units from each lot are selected and inspected. If the sample meets all requirements, the entire lot is accepted. If the selection does not complete all conditions, the lot is rejected.

Inspection sampling

Inspection sampling involves inspecting each unit in the lot. Acceptable Quality Limit tables determine if enough units should be inspected and, if so, how many.

AQL Inspection levels

Methods of Sampling

  • Single sampling: Only one sample will be taken for inspection out of the n-number samples in this sampling method. The whole batch will be rejected if any fault is identified in that sample.
  • Double sampling: This method is the continuation of 1st method. That means one more sample will be taken for inspection when the first sample fails.
  • 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. This method will take a random sample size of n from many sizes N.
  • Skip lot sampling: In this method fraction of the lots will be inspected.

Acceptable Quality Level Table

What is an AQL table?

It is a chart that shows the number of defects allowed in a product before it is considered unacceptable. In addition, the table is used to determine whether a sample of some product meets specific requirements.
It provides the minimum sample sizes needed to detect specific percentages of defects at some confidence level.

What is the Normal Acceptable Quality Level?

It is the maximum percentage of defects considered acceptable in a given product or service. The manufacturer or provider typically sets this level, which may vary depending on the product or service type. For example, a higher one may be tolerated for less expensive or more commonly available products. In comparison, a lower one may be expected for luxury items or rare items. 
A higher one may be acceptable for products used for personal use, while a lower one may be required for products used in a critical application.
Ultimately, the decision of what is considered an acceptable level of defects is up to the manufacturer or provider, and it is vital to understand the Normal Acceptable Quality Level before making a purchase.

To understand the acceptable quality level table following parameters are to be kept in mind.

  1. 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.
  2. General inspection level: It is a typical inspection level
  3. Special inspection level: It is slightly different from the general inspection level. At this inspection level, the sample size will be small compared to the general inspection level, and some special inspections to be conducted for the sample
  4. Sample size code letters: These represent the different sample size
  5. Sample sizes: It is the number of units that you take randomly for inspection
  6. Acceptable quality level: It means your tolerance for defects in a lot. Usually sets in terms of percentage
  7. Acceptance point: The maximum number of allowable defects in a given sample size.
  8. Rejection point: It is the threshold point for rejecting an order based on the defects in a given sample.

Conclusion

This post discussed an Acceptable Quality Level and how it is used with product inspection. We also went over the three categories of defects found in a lot and gave an example of how they can be expressed.

Finally, we looked at its table and discussed detecting specific percentages of defects. Hoping this post was helpful to you!