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Inspection sampling


How does sampling work during an inspection?

GIS recommends utilizing ANSI Z1.4 2008/ ANSI/ ASQC Z1.4/ BS6001/ DIN 40080/ISO 2859/ NFX 06-022(the same inspection sampling standard showed in ANSI and AQL) for inspection sampling. This standard and its equivalence are designed according to the statistics theory and are indexed with AQL(see next section) . The sampling plans in these standards indicates the number of units of products from each lot or batch which are to be inspected (sample size) and the criteria foe determination of the acceptability of the lot or batch.

Generally, inspection level Ⅱshall be used, otherwise a particular lever which determines the relation between the sample size and the lot or batch size will be prescribed by the responsible authority. Based on the lot or batch size and the inspection level code letters are assigned that cross-reference to the sample size required depending on which plan is being employed. Single and double normal sampling plans are the most commonly used. If you do not have existing sampling requirements, GIS will assist you in determining which plan works best for your specific needs. 
Samples are checked against a detailed inspection plan for appearance, applicable functionality, packaging integrity, workmanship, etc. If you currently do not have any inspection plan, GIS can assist you. 
Defects found are classified as critical, major or minor depending on the inspection plans. The acceptable quality limit (AQL) is specified by the responsible authority with the types of defects. A typical set of AQL is described in the section of “What is AQL”. Acceptability of the lot or batch is determined by the corresponding accept/reject criteria with the sampling plan.
However, it should be born in mind that this standard is intended to be used on a continuous series of lots, when used without the switching rules, the operating characteristics of the sampling plan must be assessed individually.    

What is AQL?

An AQL, or Acceptable Quality Limit, is the quality level that is the worst tolerable process average when a continuing series of lots is submitted for acceptance sampling. The AQL is a parameter of the sampling scheme. It is expected that the product quality lever will less than the AQL. There are normally three AQL specified for critical, major and minor defects separately. A typical set of AQL would be as follows:


The smaller the number, the fewer defects will be accepted in the sample of product. When choosing AQLs, it is important that one understands the balance between setting them too low, and rejecting many shipments, and setting them too high and releasing unacceptable product.

Note: AQL formerly was the abbreviation of acceptance quality level and was changed for acceptance quality limit later.

Defectives Classification

Defects is a departure of a quality characteristic from its intended level or state that occurs with a severity sufficient to cause an associated product or service not to satisfy intended normal, or foreseeable usage requirements. It can be classified as:

Critical - A critical defect is on that judgment and experience indicate is likely to:
a. result in hazardous or unsafe conditions for individuals using, maintaining, or depending upon the products; 
b. prevent performance of the tactical function of a major end item. 

Major - A major defect is one, other than critical, that is likely to result in failure, or to reduce materially the usability of the unit of product for its intended purpose. 
Minor - 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.