A reader wrote to me and asked the following question about sampling plans by attributes: What is the "Confidence Level" when using the AQL Tables? How can we determine what the confidence levels are, and are there formulas we can use? That's a valid question. When playing with statistics, one often mentions a confidence interval. For example, "90% of the time, the value will be between 34.5 and 66.0". In theory, I guess one could do the same with the values in the AQL tables. There is a … [Read more...]
A Simple AQL Calculator To Prepare Your Product Inspections
One of our developers prepared this simple AQL calculator for Sofeast. You can use it instead of reading the AQL tables to quickly produce your sample size and permissible defect number. … [Read more...]
The Special Inspection Levels in the AQL Tables
A few years ago I wrote about the inspection levels, but I regularly notice that some people are confused about the "special" levels (S1, S2, S3, and S4). Here is what the ISO 2859-1 standard says (emphasis is 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 … [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...]
Why the AQL tables are not in favor of the buyer
This is the third and last video about the ANSI tables (also known as “AQL tables”). As I explained in the last video, these sampling schemes were designed for very stable relationships, where the manufacturer keeps making the same products and keeps shipping them regularly (e.g. twice a month) to the same customer. As a consequence, the assumption is that quality is under control. And an inspection is used to catch the big accidents. What it means is that the statistics are way, way … [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 three things: The market you are selling into, The kind of risk the users run by using/consuming/getting close to your product, The amount … [Read more...]