The acceptable level is defined as the maximum percentage of defects (or the maximum number of defects per hundred units) that for the purpose of sampling inspection, can be considered satisfactory as a process average.
Acceptance sampling is actually a compromise between not doing any inspection at all and 100% inspection.
The concept of AQL only applies when an acceptance-sampling scheme is used with rules for switching between normal, tightened, reduced inspection, and discontinuance of sampling inspection.
These rules are designed to “encourage” suppliers to have process averages consistently better than the AQL. If suppliers fail to do so, there is a high probability of being switched from normal inspection to tightened inspection where lot acceptance becomes more difficult.
Once on tightened inspection, unless corrective action is taken to improve product quality, it is very likely that the rule requiring discontinuance of sampling inspection will be invoked.
Although individual lots with quality equal to the AQL are likely to be accepted, the designation of an AQL does not suggest that it is necessarily of a desirable quality level. The AQL is a parameter of the sampling scheme and should not be confused with the process average that describes the operating level of a manufacturing process. It is expected that the product defect rate will be less than the AQL to avoid having an excessive number of rejected lots.
Defects and Defectives
A defect is any single non-conformance to specified requirements that occurs in a single unit of the product. A defective is a unit of product that contains one or more defects. Failure to meet requirements with respect to quality characteristics are usually described in terms of defects or defectives.
Defects detected during visual inspection are usually classified within 3 categories which are:
A critical defect, based on judgment and experience, indicates it is likely to:
- Result in hazardous or unsafe conditions for individuals using, maintaining, or depending on the products
- Prevent performance of the tactical function of a major end item. A critical defective is a unit of product that contains one or more critical defects.
A critical defect automatically leads to the unit being rejected by the inspector.
A major defect is one, other than being critical, is likely to result in failure, or to materially reduce the usability of the unit of product for its intended purpose. A major defective is a unit of product that contains one or more major defects.
A minor defect is one that is not likely to reduce materially the usability of the unit of product for its intended purpose, or is a departure from established standards having little bearing on the effective use or operation of the unit of product. A minor defective is a unit of product that contains one or more defects.
The inspection level determines the relationship between the batch size and sample size.
There are 3 general inspection levels which are commonly used for inspections:
- Level II is designated as normal
- Level I requires about half the amount of inspection as level II, and is used when reduced sampling costs are required and a lower level of scrutiny can be tolerated.
- Level III requires about twice the amount of inspection as level II, and is used when more scrutiny is needed.
A batch is a collection of products or components of the same type and size that must meet the same specification requirements and were all produced within a related period of time. This is also known as a production run. As an example, let’s take a product that is being manufactured in five different colors. A batch of 1,000 red parts are produced by the injection molding machine and then moved to the next step in the manufacturing process. This is one batch. Next, the injection molding machine is setup to produce 1,000 blue parts of the same component. This is a separate batch. And so on.
From each batch a predetermined number of samples will be drawn and inspected for conformance to the specified requirements.
The batch size is the number of items in a lot or a batch. The words batch and lot are interchangeable.
This example illustrates the procedures for an AQL sampling inspection plan.
Product: FM/AM Auto Scan Radio
Batch size = 8,000 pieces
Method: Level II
Sample size = 200 pieces
Acceptance numbers = 10 pieces or less with a Major defect and/or 14 pieces or less with a Minor defect.
That means if either the number of Major defects exceeds 10 pieces or the Minor defects exceeds 14 pieces or both occurs the entire batch is determined to have “Failed”.
One more important point that is often overlooked by importers is the fact that the quality level of merchandise at the final destination is sometimes lower than the per-shipment results. Product deterioration can occur from transport, handling, change in environment, and/or reliability problems. You should take this into consideration when deciding AQL levels.