Which AQL do you choose?

One of the most frequent questions we receive regarding on-site quality inspections relates to selecting AQLs, or Acceptable Quality Levels.  AQLs represent the maximum percent defective that you consider acceptable.

Pro QC’s Assistant Operations Manager in China, Cesar Marsical, address the question below:

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.  Pro QC can assist with this process.

Some of the factors that must be considered prior to selecting the AQLs are:

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.

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 that you are not willing to live with.

Contact us for more info, or assistance with this process.


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