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...]
100% Inspection in a Third Party Facility in China: The How-To
I am guessing that about 96% of quality inspections done in China, on behalf of overseas buyers, are conducted in the supplier's facility on a set of randomly-picked samples. About 3% are done at the supplier's facility, on 100% of the pieces (typically, high-value items). And maybe 1% is done in a facility controlled by the buyer or a service provider engaged by the buyer. And it makes sense. In most cases, a random inspection (or a series of such inspections along the manufacturing cycle) … [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...]
When To Do a Random AQL Inspection vs. Check 100% of the Goods
First, let me get one thing out of the way: not all batches need to be checked. If a manufacturer has great systems and processes, and if they have a history of complying with your quality standard, there is usually no reason to send quality inspectors to their facility. Similarly, if you can easily return the whole batch and it can be reworked quickly, you probably don't want to spend resources on checking whether they did as promised. Now, let's assume you do want confirmation of … [Read more...]
The Different Types Of Sampling Plans For QC Inspections
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...]
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...]