Home Forums Quality Inspection Reliability of AQL in Quality Inspection

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    Jake Able

    AQL is an acronym that stands for “adequate (Acceptable Quality Limit) Sampling is a common approach for defining a production order sample in order to determine whether the complete product order meets the client’s standards. The consumer can make an informed decision to accept or reject the lot based on the sample data. Your inspection report will state if your production met or exceeded the Acceptable Quality Tolerance level you selected.
    So how reliable is the AQL calculation in sample testing and quality inspection? Can AQL fully ensure that the entire product batch is up to standard?

    Graham Steven

    The risk to the producer is that an excellent product will be rejected. The Acceptance Quality Limit (AQL) is the percentage of defective items that the plan will accept 95% of the time (in the long run). 95 percent of the time, lots that meet or exceed the AQL will be accepted (in the long run). If the lot fails, we can confidently state that the lot quality level is lower than the AQL with 95% certainty. Similarly, we can say that a lot that is approved at the AQL has a 5% probability of being rejected. The AQL in this case is 0.09 percent.

    Eddie Warren

    When working with AQL tables, how confident are you?
    A reader emailed me with the following query about sample strategies based on attributes:
    When using the AQL Tables, what is the “Confidence Level”?
    Is there a formula we can use to figure out what the confidence levels are?
    That is an excellent question. When it comes to statistics, a confidence interval is frequently mentioned. “90% of the time, the value will be between 34.5 and 66.0,” for example.
    In principle, I suppose the AQL tables’ values might be used in the same way. There is a formula (but I don’t know anyone who wants to mess about with a hypergeometric distribution – that’s some serious math).
    The statisticians who created MIL STD 105 (which later became ISO 2859-1, ASQ-ANSI Z1.4, and other standards) used a different perspective and proposed that we think about it differently.
    The operational characteristic curves – visual representations of the distribution — are included in the ISO 2859-1 standard.


    AQL in Actual Use
    Acceptable quality level (AQL): Generally speaking, AQL refers to the lowest quality level that is nevertheless regarded as acceptable. It is the highest possible percentage of defects that can be deemed acceptable. Accepting an AQL lot should be highly likely. A 0.95 probability corresponds to a 0.05 risk.
    Rejectable quality level (RQL): Also known as lot tolerance percent faulty, this term denotes an inadequate quality level (LTPD). Some tables have standardized the consumer’s risk to be 0.1. Accepting an RQL lot has a low likelihood.
    Quality level of indifference (IQL): This level of quality lies in between AQL and RQL. Each defect type is interpreted differently by various companies. But buyers and sellers settle on an AQL criteria that is suitable for the amount of risk that each side takes. During a pre-shipment inspection, these standards are utilized as a guide.

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