Using AQL, you can check whether an order meets your quality requirements without having to inspect every unit of the entire shipment, saving you time and money on . But one look at the AQL table could leave you feeling lost among the mass of numbers, letters, labels and even colors. Is this a quality control tool or a brain teaser?
Many explanations of AQL use advanced math models and equations to show how to choose a statistically significant sample for inspection. And these explanations can be hard to follow and overly complex if you’re just looking for a practical introduction on how to use AQL.
1. Acceptance sampling
Acceptance sampling is a way to pull and check a random sample of goods or material and accept or reject the total lot based on a statistical result. This lets you apply the results of inspecting this representative sample to making a decision about whether to allow your supplier to ship an order. AQL sampling is one type of acceptance sampling widely used in product inspection.
2. Acceptable quality level (AQL)
Acceptable quality level (AQL), also referred to as acceptable quality limit, is the minimum quality level a shipment is required to meet for an importer to accept it. It’s one of the most commonly used tools among quality control inspectors to measure an order’s conformance to your requirements. Acceptable quality levels typically range from 0 to 15, though AQLs higher than 6.5 are rarely used in the field. The higher your acceptable quality level, the higher your tolerance for quality defects in your order.
Quality control professionals and importers commonly use the AQL table to calculate important factors for inspection. This table tells you the appropriate number of units to pull for inspection, or your sample size, and the number of defects that would be acceptable in that sample. You can also use an online AQL calculator to find these based on your own order quantity and quality expectations.
3. ANSI-ASQ Z1.4
ANSI ASQ Z1.4 was developed by the American National Standards Institute and American Society for Quality, and it’s the most widely used standard for AQL sampling. The U.S. FDA recognizes this standard as applicable for single, double and multiple sampling plans. But most importers use it for single sampling plans. This means inspectors only pull one sample at random and issue a result based on the quality of this sample.
ANSI ASQ Z1.4 is based on inspection by attributes. This means inspected units are evaluated under two categories, either “conforming” or “nonconforming” to your requirements. Variable sampling instead measures the degree of nonconformance, based on sample standard deviation or sample range measurements. ANSI ASQ Z1.9 is the most commonly used standard for inspection by variables.
4. General inspection level
Now let’s dive into the process of how to use the AQL table for inspection. Inspection levels determine the sample size, or number of units the inspector will check on site at the factory. The bulk of your inspection will involve checks on the sample size associated with a general inspection level, like checks for appearance, assembly and functionality. In order to use the AQL table to determine your sample size, you’ll need to know your lot size, or total order quantity, typically shown on your PO.
5. Special inspection level
Special inspection levels are reserved for tests that you only intend to conduct on a comparatively smaller sample size than the general inspection level sample size. Whereas you’d use a general inspection level for tests and checks that are relatively quick and easy to perform, you’d use a special inspection level for tests that:
Are destructive to the product, like a fabric GSM test that involves cutting out a piece of fabric from the sample
Are expensive and/or time consuming to conduct, like a battery charging test for electronic devices
Tend to yield similar results across all units, like measuring the dimensions of injection-molded components that don’t vary appreciably in size from one unit to the next
Unlike the three general inspection levels, there are four special inspection levels, based on your desired inspection scope. Similar to general inspection levels, S1 is the smallest sample size for your lot size and S4 indicates the largest sample size. You might inspect a larger or smaller sample size depending on how important a test is for evaluating your product.
6. Critical defect
Critical defects are the most serious of the three main classes of defects QC professionals use for product inspection. When applying a unique AQL to each defect class, importers typically use an AQL of 0.0 for critical defects, representing zero tolerance. This means that the entire order will fail inspection if inspectors find a single critical defect in the sample.
7. Major defect
Less serious than critical defects, major defects tend to be acceptable in limited quantities but are still a concern. The most common AQL importers assign to major defects is 2.5, though you may choose a different AQL based on your own tolerance for defects. Inspectors typically consider a product defect to be “major” when it:
1). Adversely affects the performance of a product.
2). Varies significantly from product specifications.
3). Would likely cause the end customer to return the product.
8. Minor defect
Major defects are the least serious of the three defect classes. But your goods can still fail inspection if the number of minor defects found exceeds your tolerance set by AQL. An AQL of 4.0 is typical for minor defects, though you may choose a stricter or looser AQL.
9. Acceptance point
Typically denoted as “Ac” in the AQL table, the acceptance point indicates the maximum number of defects allowed in a given sample size and at a given AQL to accept an order. Just as many importers assign a different AQL to each of the three defect classes, they’ll also have a different acceptance point for each. Most importers have a higher acceptance point for minor and major defects than for critical defects, reflecting the range in AQL values they choose.
10. Rejection point
Typically denoted as “Re” in the AQL table, the rejection point indicates the minimum number of defects in a given sample size and at a given AQL to reject an order. The acceptance point and rejection point go hand-in-hand. And your rejection point will always be one value higher than your acceptance point when using a single-sampling plan.
As with using any sampling plan, your transparency and the accuracy of results are limited by the size of the sample taken. And there’s always a risk that you might reject “good” lots or accept “bad” lots when making shipping decisions based on the results of acceptance sampling.
But using an inspection method based on acceptance sampling, like AQL, ensures that you don’t inspect more goods than necessary, and in turn, don’t spend more time or money on inspection than needed. And for most importers, the relatively low risk inherent in checking a sample heavily outweighs alternative ways of inspecting their goods before shipment.