Acceptable Quality Level (AQL)

An acceptable quality level (AQL) is the percentage of defects allowed for a lot before it is rejected by a customer. Due to the size of most lots and the costs of 100% inspections, the quality of an incoming lot is frequently assessed with a sampling plan. The AQL will translate into a number of defects that are allowed in a sample of a specified size.

AQL may be negotiated in a purchasing contract, or it may simply be part of an internal quality control process.

AQL is a percentage, which translates into a specific number of defects in a particular size sample. Sampling saves cost, but has some inherent risks. Checking a sample could result in a good lot being rejected (producer’s risk, alpha risk, or Type-I risk), or a bad lot being accepted (consumer’s risk, beta risk, or Type-II risk).

A good sampling plan must be statistically sound, meaning it has a large enough sample to be representative of the actual lot within a defined confidence level, frequently set at 95%. In effect, the sampling plan says that if the inspection of X number of random units yield Y defects or less, you can be 95% sure that the entire lot meets the acceptable quality level.

Some people have a fundamental problem with defining an acceptable quality level, as it implies that a certain level of defects is OK, which is contrary to Lean’s goal of zero defects. In fact, in the Lean community, there is an anecdote about this situation.

A US customer early in the explosion of Lean was considering a Japanese supplier. The customer was focused on quality and insisted on setting an aggressive acceptable quality level. It worked out to 2 parts per lot that could be defective. The Japanese vendor initially pushed back at setting an AQL, but eventually relented and signed the contract.

When the first order arrived, there was a separate package within the main shipping carton containing two units. Attached was a note stating something to the effect of, “Here are your two defective parts. We are still not sure why you want them.”

The story highlights the differing views about quality. With strong Lean systems in place, you can virtually remove the need for establishing an acceptable quality level.