AQL stands for acceptable quality level. It is the worst average level of defects, expressed as a percentage, that you are prepared to tolerate from a supplier over a long series of lots. An AQL of 2.5 means that if the supplier consistently ships about 2.5% defective, most lots will pass. It is a boundary on the process average, not a promise about any single lot.
A common misreading is that an accepted lot contains no more than the AQL percentage of defects. It does not. A single lot can be somewhat worse than the AQL and still pass by luck of the sample, and a lot slightly better can occasionally fail. Sampling trades certainty for speed, and the AQL is the level of quality the plan is designed to let through most of the time.
Sample size depends on lot size, but not in proportion. Larger lots get larger samples, yet the sample becomes a smaller share of the lot. That is deliberate: the confidence a sample gives depends mostly on how many units you inspect, not on what fraction of the lot they represent. Inspecting 10% of a 20,000-shirt lot would be wasteful; inspecting 10% of a 100-shirt lot would be too little.
Standard Procedure
- 1Agree the AQL for each defect class with the customer before production starts.
- 2Determine the lot size from the delivery paperwork.
- 3Read the sample size that corresponds to that lot size in the agreed plan.
- 4Remember the sample share falls as lots grow; do not inflate samples to a fixed percentage.
Lesson Summary
- ✓AQL describes the worst tolerable average over a series of lots.
- ✓Sample size rises with lot size but the percentage inspected falls.