‘AQL’. stands for ‘Acceptance Quality Limit’, and is defined as the “quality level that is the worst tolerable” (ISO 2859-1 standard).
Acceptable Quality Limit (AQL) is the standard used by many Quality Control Inspection firms and inspectors to determine a proper sampling size and the number of allowable defects. AQL is a statistical measurement of the maximum number of defective goods considered acceptable in a particular sample size.
History of AQL
Prior to the mid-1900s, acceptance sampling was not widely followed. Acceptance sampling was popularized by Dodge and Roming (pioneers of modern Quality Control) and was originally applied by the U.S. military for testing bullets during World War II. The dilemma was, if every bullet was tested in advance, no bullets would be able to ship on time. If, on the other hand, no bullets were tested, malfunctions might occur on the battlefield, with potentially catastrophic results.
Based on the extensive work by the American military during and past world war II, US Government issued the standard for sampling procedure and tables for inspection called MIL-STD-105D in 1963.This was further modified in 1989 as MIL-STD 105 E and re-designated as ANSI/ ASQC Z 1.4 in Feb 1995. For all the practical purposes MIL STD 105D and ANSI/ASQC Z 1.4 are almost similar.
Use of AQL
It becomes necessary to maintain the quality right from the beginning in order to deliver a satisfactory final product to the consumer. An acceptable quality level is a test and/or inspection standard that prescribes the range of the number of defective components that is considered acceptable when random sampling those components during an inspection.
The most commonly used standard for product inspections is standard ISO 2859-1 (ANSI/ASQC Z1.4-2003). It uses the concept of AQL, or Acceptable Quality Limit.
Acceptance sampling plans help in distinguishing between the acceptable and the unacceptable lots. A sampling size, based on the AQL tables, will be selected and then inspected for defects.
Defects are Divided into 3 categories: Minor, Major and Critical. While these can vary from client to client, the typical definitions are as follow:
§ A Minor defect is a discrepancy from the standards, but one that is not likely to affect the usability of an object.
§ A Major defect is one that is likely to create failure of the unit for its intended purpose.
§ A Critical defect is one that is deemed to be hazardous or unsafe.
According to the number of defects found for each type and according to the number of defects allowed (figures given by the AQL tables), Inspector accept or to reject shipment.
Also Read How to use AQL table