AQL vs LTPD
Short answer
AQL (acceptable quality limit) is the defect rate that a sampling plan accepts with high probability, typically about 95 %, so the risk of rejecting a good lot (producer’s risk) is low. LTPD (lot tolerance percent defective, also called RQL) is the defect rate that the plan accepts only rarely, typically 10 %, so the risk of accepting a bad lot (consumer’s risk) is low. Both are points on the same operating characteristic (OC) curve of the plan.
Side by side
| Aspect | AQL | LTPD / RQL |
|---|---|---|
| Meaning | Quality level considered acceptable | Quality level considered unacceptable |
| Probability of acceptance | High, about 90 to 99 % | Low, typically 10 % |
| Risk it controls | Producer’s risk α (good lots rejected) | Consumer’s risk β (bad lots accepted) |
| Who cares most | Supplier | Customer |
| Standards | ISO 2859-1, ANSI/ASQ Z1.4 (lot series) | ISO 2859-2 (isolated lots), Dodge-Romig tables |
Why AQL alone can mislead
Two plans with the same AQL can give very different protection against bad lots. A small sample with acceptance number 1 may accept a lot with 10 % defects quite often. Always look at the OC curve or the LTPD of the plan before agreeing on it, especially for isolated lots where the switching rules do not help.