Due to the high costs associated with piece by piece inspections, these are generally limited to small rush orders and high value products. In most cases, it is not necessary to inspect every piece of an SKU in a lot before shipping. The industry standard for a pre-shipment inspection is to perform a check on random samples of a production lot after production is at least 80% complete and packaged for shipment.
This is a more cost-effective approach and is widely accepted for most consumer goods. Sampling rates in a quality control inspection have been established to calculate the defect rate, or defect percentage, in a production lot. It is called the Acceptable Quality Limit or Level (AQL) and is widely used as the framework for determining the appropriate sampling size (number of items checked) and ratios of defects found in the sample size.
What is AQL?
The AQL standard is defined in ISO 2859-1 as “the quality level that is the worst tolerable percentage or ratio of defects that are still acceptable”. It represents the maximum number and type of defects that can be accepted in an inspected example, beyond which the entire batch must be rejected.
AQLs can be set for a percentage or number of critical, major and minor defects. In practice, these three levels are most often used, and especially for consumer products. They are defined as follows:
- Critical Defects – 0%
Not acceptable as a user could be harmed, the product is not compliant with regulations, or the defect results in product failure.
- Major Defects – 2.5%
This represents defects that result in the product being considered unacceptable by the end user.
- Minor Defects – 4.0%
Failure to meet specification in ways that most buyers would still accept.
The most common AQL standard for the consumer goods industry was developed by the American National Standards Institute (ANSI) and the American Society for Quality (ASQ). AQL table is used as a fundamental tool for preparing a sampling plan for random inspections of product .
Table 1 – Sample size code letters
Table 2 – Single sampling plans for normal inspection
Notes: “√” means acceptance “×” means rejection
“↑ “ :use first sampling plan above arrow. If sample size equals or exceeds lot or batch size, do 100% inspection.
“↓ “: use first sampling plan below arrow.
Imagine you have an order of 960 articles of clothing ready to ship. How will you calculate the standard AQL required for the inspection?
- Check Table 1 for general inspection levels and select your lot size in the relevant range, which is 501-1200. If you run your inspection at the average inspection level II, your code would be the letter J.
- On Table 2 locate the code letter J and , and you can see your sample size is 80, which indicates the number of pieces required to be inspected.
Determine the number of defects
Based on the sample size above, you still need to determine the allowable or maximum number of critical, major and minor defects to pass the inspection. AQL 0.0 is for critical defects, AQL 2.5 for major defects, and AQL 4.0 for minor defects.
To determine the number of allowable major and minor defects, follow the J row to the 2.5% column where the numbers 5 and 6 are recorded. The lower number, 5, is the maximum number of allowable defects that will result in acceptance. In the same way, you can find the number 7 as the maximum number of allowable defects for minor defects.
AQL Summary Table
|AQL Summary for 960 Articles|
|Level||Sample||AQL Critical||AQL Major||AQL Minor|
General and Special inspection levels
Special inspection levels (S-1, S-2, S-3, S-4) are normally used for certain types of on-site inspections or checks that are only necessary on a relatively small number of units in the lot. The results are not likely to differ significantly between units inspected (often <10 units), and especially where an inspection requires destructive or time-consuming tests.
General inspection levels (G-I, G-II, G-III) are the most common inspection levels used for larger lots. These cover all the regular checks and tests that can be conducted relatively quickly with varying results across all items sampled. They are generally used in the following way:
Products with less strict requirements
The most common sampling plan for consumer products under normal conditions.
Products with more strict requirements.
AQL sampling and QC inspection with HQTS
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