1. What is a sampling plan? A sampling plan allows an auditor or a researcher to study a group (e.g. a batch of products, a segment of the population) by observing only a part of that group, and to reach conclusions with a pre-defined level of certainty. In this article I cover these cases in depth (hit the links below to skip to the relevant section): The buyer wants to follow the most common plan to inspect a supplier's batch of products The manufacturer implements a continuous … [Read more...]
What is the “AQL” (Acceptance Quality Limit) in simple terms?
What Does AQL Mean? 'AQL' stands for 'Acceptance Quality Limit', and is defined as the "quality level that is the worst tolerable" in ISO 2859-1. It represents the maximum number of defective units, beyond which a batch is rejected. Importers usually set different AQLs for critical, major, and minor defects. Most Asian exporters are familiar with this type of setting. For example: "AQL is 1.5%" means "I want no more than 1.5% defective items in the whole order quantity, on average over … [Read more...]
The Different Sampling Plans Contained in the ISO 2859 Series
The vast majority of inspectors follow the ISO 2859-1 standard. But about ISO 2859-2, -3, and so on? The relationship between these parts is explained in ISO 2859-10:2006 (introduction to the ISO 2859 series of standards for sampling for inspection by attributes): ⎯ Part 1: Sampling schemes indexed by acceptance quality limit (AQL) for lot-by-lot inspection ⎯ Part 2: Sampling plans indexed by limiting quality (LQ) for isolated lot inspection ⎯ Part 3: Skip-lot sampling procedures ⎯ … [Read more...]
The Special Inspection Levels in the AQL Tables
A few years ago I wrote about the inspection levels, but some people are still confused about the "special" levels (S1, S2, S3, and S4). Here is what the ISO 2859-1 standard says (emphasis mine): The inspection level designates the relative amount of inspection. Three inspection levels, I, II and III, are given in Table 1 for general use. Unless otherwise specified, level II shall be used. Level I may be used when less discrimination is needed or level III when greater discrimination is … [Read more...]
How to choose an AQL limit?
A few months ago, I wrote an explanation about the "AQL": what it is and how to use it. I also listed a few frequent questions, but it seems like I forgot one of them: What AQL tolerance is suitable to my products? Unfortunately, it is hard to base this on statistical reasoning. So I don't have any definitive answer. The right AQL depends on two things, I think: The market you are selling into, The kind of risk the users run by using/consuming/getting close to your product. 1. … [Read more...]