AQL is short for Acceptable Quality Level, or the worst possible quality accepted for a given product or service. The origin of AQL dates back to World War 2 where sampling was used to inspect bullets.
If you have been involved in any quality control activity, chances are you have come across the AQL term, as it is one of the most commonly used tools in product inspection.
Below are the two tables used to determine the required sample size and acceptable amount of rejects.
For table 01 the lot size is seen on the left, and inspection level at the top. For example is we have received a delivery of 2500 parts and need to do an AQL inspection, we would look at the row indicating a lot size between 1201 and 3200.
For the inspection level, the most commonly used is general inspection level 2. Using the example of 2500 parts, this will give us the letter K according to the table.
For table 02 we can see the sample size to be inspected is 125 pcs based on the K we got from the first table. In order to see what the acceptable amount of defects is, the acceptable quality level need to be defined. If AQL 0.025 is used in our example, 1 reject would result in rejecting the entire lot. But if AQL 0.65 is used, we would not reject the lot unless 3 or more of the 125 pcs inspected parts are nonconforming.
Criticism of AQL
There are different opinions about the use of acceptable quality level, both for and against. One of the common heard criticism of the system, is that it is like telling a supplier they can ship x amount of rejects for every 1000 parts they send you. When looking at the quality culture in Japan compared to western industries, they don’t understand the concept of being acceptable to ship defects.
If implementing the use of AQL, it should be clear it is used as a reference rather than a tool to determine how much scrap can be send. Otherwise you risk having some suppliers deliberately adding “acceptable” level of defects to deliveries.
If you are interested in AQL, or need training for your employees in sampling for quality control, then contact us for a free consultation.
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