AQL Inspection Manual
Acceptance Number. The acceptance number is the maximum number of defects or defective units in the sample that will permit acceptance lot or batch.
MIL-STD-105E, ISO 2859-1 (1999)
Acceptable Quality Level. The acceptable level (AQL) is defined as the maximum percent defective (or the maximum number of defects per hundred units) that, for purpose of sampling inspection, can be considered satisfactory as a process average. The sampling plans most frequently used by the department of Defense are based on the AQL.
Acceptance Quality Limit. The AQL is the quality level that is the worst tolerable process average when a continuing series of lots is submitted for acceptance sampling.
The following note on the meaning of AQL was introduced with the ANSI/ASQ Z1.4-2003 revision.
The concept of AQL only applies when an acceptance sampling scheme with rules for switching between normal, tightened and reduced inspection and discontinuance of sampling inspection is used. These rules are designed to encourage suppliers to have process averages consistently better than the AQL. If suppliers fail to do so, there is a high probability of being switched from normal inspection to tightened inspection where lot acceptance becomes more difficult. Once on tightened inspection, unless corrective action is taken to improve product quality, it is very likely that the rule requiring discontinuance of sampling inspection will be invoked.
Although individual lots with quality as bad as the AQL can be accepted with fairly high probability, the designation of an AQL does not suggest that this is necessarily a desirable quality level. The AQL is a parameter of the sampling scheme and should not be confused with a process average which describes the operating level of a manufacturing process. It is expected that the product quality level will be less than the AQL to avoid excessive non accepted lots.
The AQL values are defined as percent nonconforming or defects or nonconformities per hundred units.
Defects and Defectives. A defect is any nonconformance of the unit of product with the specified requirements. A defective is a unit of product which contains one or more defects. Failure to meet requirements with respect to quality characteristics are usually described in terms of defects or defectives.
Critical - A critical defect is on that judgment and experience indicate is likely to:
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. A major defective is a unit of product that contains one or more major defects.
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. A Minor defective is a unit of product that contains one or more defects.
Double Sampling Plan. A double sampling plan involves sampling inspection in which the inspection of the first sample to a decision to accept, to reject or to take a second sample. The inspection of a second sample, when required, lead to a decision to accept or reject.
Drawing of Samples. Basic to sampling inspection is the assurance that the sample selected from a quantity of units represents the quality of that quantity of units. Hence, the procedure used to select units from a lot must be such that it assures a sample free of bias.
Expression of Nonconformance. The extent of nonconformance of product shall be expressed either in terms of percent defective or in terms of defects per hundred units (DHU).
Defects per Hundred Units. The number of defects per hundred units of any given quantity units of product is one hundred times the number of defects contained therein (one or more defects being possible in any unit of product) divided by the total number of units of product, i.e.:
Inspection. Inspection is the process of measuring, examining, testing, or otherwise comparing the unit of product with the requirements.
Inspection by Attribute. Inspection by attributes is inspection where by either the unit of product is classified simply as defective or non-defective, or the number of defects in the unit of product is counted, with respect to a given requirement or set of requirements.
Inspection Levels. The standards provides for three general inspection levels and four special inspection levels. These seven levels permit the user to balance the cost of inspection against the amount of protection required.
Lot or Batch. The term lot or batch shall mean "inspection lot" or "inspection batch" i.e., a collection of units of product from which a sample is to drawn and inspected to determine conformance with the acceptance criteria, any may differ from a collection of units designated as a lot or batch for other purposes (e.g., production, shipment, etc.).
Nonconformance. Nonconformance may be defined as the failure of a unit of product to conform to specified requirements for any stated quality characteristic. The extent of nonconformance of product to the required quality characteristics shall be expressed either in terms of percent defective or in terms of defects per hundred units (DHU).
Normal Inspection. Normal inspection is that which is used where there is no evidence that the quality of product being submitted is better or poorer than the specified quality level.
Percent Defective. The percent defective of any given quantity of units of product is one hundred times the number of defective units of product contained therein divided by the total number of units of product, i.e.: Percent defective = number of defectives x100 / number of units inspected
Reduced Inspection. Reduced inspection under a sampling plan uses the same quality level as for normal inspection, but requires a smaller sample for inspection.
Rejection Number. The rejection number is the minimum number of defects or defective units in the sample that will cause rejection of the lot represented by the sample.
Representative Sampling. When appropriate, the number of units in the sample shall be selected in proportion to the size of sub-lots or sub-batches, or parts or the lot or batch, identified by some rational criterion. When representative sampling is used, the units from each part of the lot or batch shall be selected at random.
Resubmitted Lots or Batches. Lots or batches found unacceptable shall be resubmitted for reinspection only after all units are re-examined or retested and all defective units are removed or defects corrected. The responsible authority shall determine whether normal or tightened inspection shall be used and whether reinspection shall include all types or classes of defects or only the particular types or classes of defects which caused initial rejection.
Sample. A sample consists of one or more units of product drawn from a lot or batch, the units of the sample being selected at random without regard to their quality. The number of product in the sample is the sample size.
Single Sample Plan. A single sampling plan is a type of sampling plan by which the results of a single sample from an inspection lot are conclusive in determining acceptability. The number of sample units inspected shall be equal to the sample size given by the plan.
Severity of Inspection. The severity of inspection concerns the total amount, kind and extent of inspection specified by the quality assurance provisions established for the unit of product, or as dictated by quality history.
Unit of Production. The unit of product is the thing inspected in order to determine its classification as defective or non-defective or to count the number of defects. It may be a single article, a pair, a set, a length, an area, an operation, a volume, a component of an end product, or the end product itself. The unit of product may or may not be the same as the unit of purchase, supply, production, or shipment.
Tightened Inspection. Tightened inspection under a sampling procedure plan uses the quality level as for normal inspection, but requires more stringent acceptance criteria.