Some of my new clients, when they book their first inspection, tell me “please check 10% of the order quantity”. Here is what I typically respond:
Usually we follow the statistical tables that everybody in the industry works with. They were developed specifically for quality control, and they have become a standard that even your suppliers have heard of. Nearly 100% of importers follow this standard.
But we certainly can apply your inspection plan if you explain it to me. If we randomly select 10% of the order quantity for checking, what is the maximum number of major and minor defects that you will accept?
Generally this is enough to convince them to follow the relevant standards. But some of them actually stick to their guns, and give me an upper limit on the number of defects. In this case, here is what I say:
We can follow your suggestion. But in case the inspection is failed, it will be easy for your supplier to question the scientific validity of the inspection plan.
The maximum number of defects you gave me does not take into account the risk that the whole quantity is acceptable while the samples we have selected are worse than the average. Neither have you considered the risk of the opposite happening (accepting a lot because of samples above average). You would need to choose a confidence level, calculate the impact of both risks, and come up with limits on the numbers of defects.
Then they realize they are better off not reinventing the wheel. We are also better off, as a new inspection plan creates confusion and can lead inspectors to make mistakes.
Another classic is trying to “optimize” the work done. Let’s say the product is rather simple, and an inspector can check up to 315 samples of in one day. The order quantity is 3,000 pcs, so even a level III inspection would require us to draw “only” 200 samples. A buyer willing to optimize the inspector’s job is likely to ask us to check 315 samples anyway. Once again, the statistical tables don’t allow for this situation.
For 90% of consumer goods (excluding food) sourced in China, following the “general level II” is fine. Instead of inventing their own procedure, importers would be better inspired to spend more time communicating with their suppliers and preventing mistakes.
To learn more about the statistics behind QC inspections, you can check out this article: What is the AQL?