What are AQL tables?
AQL stands for Acceptance Quality Limit and it is defined as “quality level that is the worst tolerable” as per ISO standards. In production industry, one has to give exact number as to how much defective items are tolerable during the whole production cycle of the order and what kind of defects are termed as critical, major, and minor.
AQL tables helps the quality check people to keep the objectives clear, easy to interpret and read. Based on this statistical data the production can be accepted or rejected.
Secret Code letters
Code letters denote the sampling size and the maximum number of defects allowed in that given sample. Code letter is selected on the basis of lot size and inspection level from the AQL table.
Sample table for selecting the code:
From the above table, it is clearly visible that for lot size between 3,201 to 10,000 pieces and level II general inspection one has to refer to code ‘L’.
Sample table for code interpretation:
In the event, the selected limit is set for 2.5% for major defect and 4.0% for minor defect, than from 200 samples if more than 10 samples have major defect and/or 14 or more samples have minor defect the production batch is rejected.
Sometimes from single sample plan, no clear judgments can be made whether to reject or accept the production. In that case, one has to go for double sampling plans. This gives clearer picture of whether to accept or reject the production based on the quality inspection.
Sample table for Double sampling:
For reading purposes the down arrow (↓) indicates follow the sample plan right below the arrow and up arrows (↑) indicates that one has to follow the sample plan right above the arrow.
When and where double sampling is needed?
In the situations where the rejection and acceptance limit are vague or are not defined clearly second sampling is required. For example in the situation with the code L in general inspection level II with 1.0% limit and first sample limit as 125 — the acceptance limit is 2 and rejection limit is 5.That means if there are 2 or less than two goods with minor defects the production is accepted and if the defective products are 5 the batch is rejected. Now the question arise, what if the defective products are 3 or 4 from the given sample. The table doesn’t state whether to reject or accept. In that case second sampling is done with same sample limit as 125 and numbers of defective goods are counted again. In case total defective products are less than or equal to 6 the batch is accepted but in case total defective products are 7 or more the batch is rejected.
Note: The total numbers of defective products are sum of defective products from both the sample batch not just the second sample. Thus the number 6 is total number of defective products from sample1 and sample2 (250 samples that were inspected).
Secondly the sample limit for both the sampling round should be same. One cannot use 150 samples the first time and 100 samples the other time to calculate the acceptance or rejection of the batch.
One has to select the sampling style based on the needs and make sure to follow the AQL tables correctly as it is a well devised statistical tool.