The Truth: Acceptance Quality Limit (AQL) sampling inspection is the most widely accepted sampling inspection protocol for inspection by attribute. The past specifying of AQL plans by the U.S. Department of Defense has greatly increased the role of this method of sampling. The original MIL-STD-105A standard for sampling inspection was developed by the U.S. Military in 1950. The standard underwent several modifications and became MIL-STD-105E in 1989. The Department of Defense discontinued the MIL-STD-105E standard in February 1995 and users were referred to its commercial equivalent ANSI/ASQC Z1.4-1993
The ISO standard for inspection by attributes was introduced in 1989 as the ISO 2859-1:1989 standard matching the ANSI/ASQC Z1.4-1993 and MIL-STD-105E standards. The ISO standard was then modified including changes to its sample plans in a 1999 revision. The ANSI/ASQ Z1.4-2003 standard revision was released in January 2004.
Note: KRT Audit Corporation inspects according to the ANSI/ASQ Z1.4-2003 standard and the ISO 2859 standard, both of which have since replaced the MIL-STD-105E standard.
AQL: AQL has been changed from “Acceptance Quality Level” to “Acceptance Quality Limit”. It actually has TWO different definitions due to standard changes. Here is the definition according to MIL-STD-195E and ISO 2859-1 (1999: “The acceptable quality limit (AQL) is defined as the maximum percent defective (or maximum number of defects per hundred), that, for purpose of sampling inspection, can be considered satisfactory as a process average. However, the ANSI/ASQC Z1.4-2003 states: “The AQL is the quality level that is the worst tolerable process average when a continuing series of lots is submitted for acceptance sampling.
Defects/Defectives: A “defect” is any nonconformance of the unit of product against the specified requirements. A “defective” is a unit of product which contains one or more defects. Below are the different defect level definitions:
Critical Defect: A “Critical” defect is on one’s judgment and experience indicates is likely to: (1) Result in hazardous or unsafe use, operation, or maintenance of the product, or (2) Prevent performance of the tactical function of a major end item.
Major Defect: A “Major” defect is one other than critical, that is likely to result in the failure or to reduce materially the usability of the unit of product for its intended purpose.
Minor Defect: 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.