GUEST POSTING: Quality Control Basics, Part 1/4: Inspection Levels
This series of articles is about QC inspections. Maybe you let your supplier ship the goods without inspecting. Maybe you use a third-party company to control your products. Maybe you have your own inspectors. Are you taking unnecessary risks? Are you paying too much? The only way to form an opinion about these questions is to be familiar with the basics of quality control.
A word about applicable standards
Militari Standard 105 was created by the US Department of Defense to control their procurements more efficiently. In 1994 they decided to rely on non-governmental organizations to maintain this type of standard. The ANSI, ISO, and other institutes all created their own standard, but in essence they are similar to the latest version of Mlt-Std 105. All third-party QC firms use the same standards and the same statistical tables.
Why use random sampling?
Shipments often represent thousands of products. Checking 100% of the quantity would be long and expensive. A solution is to select samples at random and inspect them, instead of checking the whole lot. But how many samples to select? On the one hand, checking only a few pieces might prevent the inspector from noticing quality issues; on the other hand, the objective is to keep the inspection short by reducing the number of samples to check. The relevant standards propose a standard severity, called “normal level” or “level II”. It is designed to balance these two imperatives in the most efficient manner, and it is used for more than 90% of inspections. For example, for an order of 8,000 products, only 200 samples are checked.
When to switch to tightened or reduced levels?
Suppose you source a product from a factory that often ships substandard quality. You know that the risk is higher than average. How to increase the discriminating power of the inspection? You can opt for the “tightened level” (level III) and more samples are checked. Similarly, if a supplier has consistently delivered acceptable products in the past and keeps using the same workshop, you can choose the “reduced level” (level I). As fewer samples have to be checked, the inspection might take less time and be cheaper. In practice, the relevant standards give very precise guidelines about when to switch, but most importers rely on their “gut feeling”. If you want to respect these guidelines strictly, ask your QC manager or your external inspection provider.
The “special levels”
Inspectors frequently have to perform some special tests on the products they are checking. In some cases the tests can only be performed on very few samples, for two reasons:
- They might take a long time (e.g. doing a full function test as per claims on the retail box).
- They end up in product destruction. (e.g. unstitching a jacket to check the lining fabric). For these situations only, the inspector can choose a “special level”.
So we have three “general” inspection levels, and four “special levels”. For a given order quantity, each level gives a different number of samples to check. Let's see how it plays out in two examples.
Example 1: You order 40,000 products
Example 2: You order 3,000 products
The number of samples to draw from varies from 8 to 800. Depending on the level you choose, the inspection might take only one day, or up to 4 or 5 days.
In practice, how to know the number of samples to select for each order quantity and each level? In the next article I will show how to read the statistical tables and get this information.
The number of samples to draw from varies from 5 to 200. If the product is not particularly complex, a professional inspector can check 200 samples in a day. In this case you can choose the tightened level for more reliability at no extra cost. But the factory might have a little more repackaging work.
Renaud Anjoran is the founder of Sofeast Quality Control, a third-party QC firm specializing in garments and textile in China. He also writes on the Quality Inspection blog. You can contact him at email@example.com.
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