- July 6, 2017
- Posted by: admin
- Category: Quality Control News, Quality Inspection
What’s AQL table
‘AQL‘ stands for ‘Acceptance Quality Limit‘, and define as the “quality level that is the worst tolerable” in ISO 2859-1. The AQL table will help to determine the sampling size we will need to inspect according to your ordered quantity and your level of severity (I, II or III). The standard level, 98% of the people is using the level II for a standard inspection.
FAQ 1: Should i have to authorize the factory to produce some defects?
>> In theory no. That’s why the AQL(How to use AQL Table) renamed from “acceptable quality level” to “acceptance quality limit”. It is a “limit” (and a loose one at that).
Refer to what the ISO2859 Sampling Procedures for Inspection standard says:
“Although individual lots with quality as bad as the acceptance quality limit may be accepted with fairly high probability, the designation of an acceptance quality limit does not suggest that this is a desirable quality level. Sampling schemes […] are designed to encourage suppliers to have process averages consistently better than the AQL.”
So please note that using these statistics means you assume the factory cannot to produce goods with 100% good quality, in practice.
Alternatively, you can choose an “acceptance on zero” plan. As soon as the inspector found one defect, the inspection failed. But you can impose this to suppliers only in situations where quality requirements are very high (in the auto industry or in aerospace…).
FAQ 2: As a buyer, should I charge back the value of the defective products to the manufacturer?
>> This is not standard practice. The standard practice is actually to charge nothing back, as long as the inspection passed. But then, if the inspection failed, the supplier has to sort & rework the goods, and submit them to a new inspection (and the re-inspection costs are charged back to them).
FAQ 3: Based on my quality level, I calculated the proportion of defects authorized. Why don’t they correspond to the maximum number of defects authorized?
>> It is true. for example, 2.5% of 200 samples is 5 samples, but we accept the goods even if 10 samples are found with a major defect.
Why this difference? There are heavy statistics behind this issue. To keep it simple, the producer’s risk is his risk of rejection (based on the random element when drawing the sample) even though his products (if they were all checked) would be accepted. That risk is about 5% in this standard. And, along the same logic, there is a consumer’s risk and is is around 10%. As you can see, this standard is favorable to the producer’s side.