Question
I need help understanding the AQL values in the tables of ASQ Z1.4. They are defined in paragraph 4.5 as percentages or ratios, but there are some values that are less than 1 and greater than 100. How should these values be interpreted? Since this standard is for attribute data, is there a standard for variable data?
Answer
A percentage can be from 0 to more than 100% depending on what the ratio represents. First we need to define AQL. Section 4.2 states “The AQL is the quality level that is the worst tolerable process average when a continuing series of lots is submitted for acceptance sampling.” Therefore, an AQL of 0.65% means that on average we can accept 65 defects per 10,000 units in a lot. The sampling plans with percentages greater than 100% are carried over from the MIL-STD-105 and are considered to be antiquated and not used any longer.
The ANSI standard for variable data sampling plans is ANSI/ASQ Z1.9. It is based on probability of being outside the acceptance region.
For more on this topic, please visit ASQ’s website.