Why AQL .65 Critical and 1.5 Cosmetic?

QElifer

Registered
Hi, are there any standards or requirements that address what AQL (ANSI tables, etc.) is "required" when using sampling tables? Example: It seems the default (where I've worked) is .65 for critical dimensions and 1.5 for cosmetics for checking lots at various production points. My boss wants to know why we couldn't change to 1.5 or even 2.5 for Critical if we "wanted to" other than maybe modifying validations, etc. We make a class two implant so I'm personally horrified but he's fond of "okay, but just curious, why, who says" Thank you.
 

Bev D

Heretical Statistician
Leader
Super Moderator
Lots to unpack in this one.
First a class 2 implant device is nothing to be cavalier about.

Second: even a 0.65 AQL is fairly high for a critical dimension for this type of device. As a reminder the AQL is the ACCEPTABLE defect rate. So a 1.5 or 2.5 AQL would mean that you guys are OK shipping 2.5% defective product on a regular basis.

Now of course there is another question that might come into play here and that is how the critical dimensions are determined. IF the design engineers just wing it thru their engineering judgment then maybe your boss is just questioning the engineer’s choices? IF they are empirically determined through engineered specifications then your boss has no grounds to question this unless he can prove that the engineers are wrong.

Third: in many industries the AQL is defined by the Customer. In this case since you are the end producer (correct me if I’m wrong) then your label claim and regulatory agency would state the defect rate. (I have to say that usually in these cases it is the RQL that matters and not the AQL, too many people - even statisticians at the regulatory agencies - don’t seem to understand the difference). Clearly your boss either doesn’t understand simple statistics and/or doesn’t care about the patients that will be effected by your defects. Your inspection sample sizes should protect the user in alignment with your validations and claims.

Fourth and maybe the most important item to clarify is that you say this is for “checking lots at various production points”. AQL/RQL inspection plans are used at incoming inspection and final release testing. Testing at various production points is usually a small sample that is used to ensure that natural changes in the process are still producing conforming parts. Changes range from material lot changes, set-points in processes, operator changes…Often the sample size here can be from 1-5, especially if you employ SPC or even PreControl (don’t knock it til you really understand it).

So in order to answer your boss we would need to understand this last point as well the link between your validations/claims and your choice of sample size.
 
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