Let’s say you want to follow the general procedure for quality inspections. How many samples should you draw for inspection?
Let’s also say you want to follow the ISO 2859-1 standard, under normal severity, and in level II. (If you are not familiar with these concepts, you can read the articles listed under Quality Control Basics).
It is fairly easy to read the AQL tables and to see what the number of samples to inspect is…
… Except in certain situations, in particular when your AQL limits are low and your sample size is low. You might have to select different numbers of samples for each category of defect.
Let’s walk through two examples to illustrate this.
Order quantity is 20 pieces. Level II, normal severity. AQL is 1.5 for major, and 2.5 for minor.
The letter is C:
The inspector draws 8 samples at random, and the batch is rejected if he finds at least 1 defect in any of the 2 categories. It means there are 3 samples that are checked for major defects but that are not checked for minor defects.
Order quantity is 30 pieces. Level II, normal severity. AQL is 2.5 for major, and 4.0 for minor.
The letter is D:
The inspector draws 13 samples at random. For major defects, only 5 samples are checked, and the batch is rejected if 1 or more defects are found. For minor defects, 13 samples are checked, and the batch is rejected if 2 or more defects are found.
This is one of the details that many people in the industry tend to overlook. Yet it is indispensable if one is to apply these statistical rules correctly.
On a similar subject, you can read Statistical inspection levels: beware of the confusion.