Acceptable Quality Level
Acceptable Quality Level (AQL); used in sampling inspection, the maximum percentage of defectives that is acceptable as a long-term average. See also OC Curve.
See Acceptable Quality Level.
Used in sampling inspection. It is the risk that a consumer will accept a batch of worse quality than the Limiting Quality Level. It is equivalent to a Type II Error. See also OC Curve.
|Limiting Quality Level
LQL. Used in sampling inspection. It is the proportion of nonconforming items associated with the consumer’s risk. It can be regarded as the minimum quality that the customer would not want to accept, even for a single batch. The AQL is the minimum acceptable long term average. See also OC Curve.
See Limiting Quality Level.
Lot Tolerance Percent Defective. The Limiting Quality Level expressed as a percentage.
Curves that show the probability of accepting a batch over a range of proportions of nonconforming items. The figure shows an 'OC' (Operating Characteristic) Curve for a sample if 50 taken from a batch of 2000 and using a critical acceptance number 'c' of 2.
Note that OC Curves are also used other than in sampling inspection. They are used to find the Average Run Length in control charts.
|Operating Characteristic Curve
See OC Curve.
Used in sampling inspection. It is the probability that a batch with a quality equal to, or better than, the AQL will be rejected. It is equivalent to the Type I error. See also OC Curve.
To avoid calculation, and negotiation, published standard sampling plans are normally used for inspection. The most commonly accepted being ANSI/ASQ Z1.4-2003.
The figure shows an example of a page from a sampling plan:
The first step is to find the sample size code letter. This is obtained from a separate table and is based on the lot size and the severity of the inspection required. A value for the AQL is also selected.
The table shown above is then used to find the sample accept/reject values. For example if the code letter were 'N' and the AQL was 0.15 then the sample size would be 500. The batch would be accepted if there were 1 defect or less, and rejected if there were two defects or more.