How do you select AQL levels?

2013-03-07

Written by: Cesar Marsical, Assistant Operations Manager To define AQL, it is important to note there are two different explanations due to standard changes. Under MIL-STD-105E, ISO 2859-1: AQL - 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. Under ANSI/ASQ Z1.4-2003/2008: AQL - Acceptance Quality Limit The AQL is the quality level that is the worst tolerable process average when continuing series of lots is submitted for acceptance sampling. The selection of AQLs and sampling plan for a given lot size depends on too many factors to permit the issuance of a "pre-selected" standard set of plans for specified lots. Each user should select AQLs and sampling plans that are tailored to best meet their needs. Some of the factors that must be considered prior to selecting the AQLs include: 1) Classifications or categories of defects such as Critical, Major and Minor Critical defects would generally require zero defects, which means the highest AQL value should be imposed. Major defects would generally require a lower AQL than those for minor defects. Pro QC has standard defect classifications posted to here: http://proqc.com/products-defects-classification.php 2) Process capabilities under good commercial practice with respect to the defects in question. For example, if under normal production process, the defect levels cannot be kept below 2.0 percent defective, the selection of an AQL of 0.15 percent defective, although desirable for the defects in question, may not be practical. 3) Consumer preferences- These may require higher AQL's or permit lower AQL's than process capabilities would indicate. 4) Time and cost required to sample and inspect a lot under various AQLs - The smaller the AQL, the more time and cost of inspection. Some of the factors that may be considered prior to selecting the sampling plans for a given lot size includes: 1) The applicable AQLs - The AQL dictates, among other things, the smallest sample size that can be used and the size of the "jumps" from one sample size to the next larger one. 2) The relative ability of the plans to discriminate between "good" and "bad" lots. Although several plans in these standards have the same AQL, they differ in their ability to reject lots worse than the AQL's. The OC (Operating Characteristics) curve in the standards of this subpart provides the basis for determining the discriminating ability of each plan. 3) The amount, time, and cost of sampling required. 4) The size and value of the lots relative to the producer and consumer protection a sampling plan affords. One may be willing to take larger risks of passing "bad" lots that are small or of lesser value than they would for larger more valuable lots. 5) The knowledge about the lot(s) to be submitted for inspection. Lots consisting of product produced under essentially the same conditions may require smaller sample size than those consisting of product produced by different shifts and different raw stocks for example. 6) The record of the quality level of previously submitted lots. The sample size can be smaller for lots submitted from a supplier with a consistent record of quality levels significantly better than the specified AQL(s) than sample sizes for the supplier whose records shows considerable variability in quality, "borderline" supplies or products worse than the AQL. Based on the above, decide what quality levels you are willing to live with and what quality level you are not willing to live with. Contact your account manager for additional information. Pro QC can assist with this process. The ANSI Z1.4 sampling tables are posted to the Pro QC website here: http://proqc.com/ansi-sampling-tables.php

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