# Where does 20% Lot Qualification Rule Come from?

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#### rpanciocco

I've been in med device for awhile now and worked a few different places. I keep running into inspection plans that call for 20% of the lot to be inspected. Typically, the company follows the 20% rule without process validation being done at all - as if 20% inspection of the lot somehow negates the need to validate the process. But, when I ask why 20% is considered sufficient since the process isn't validated, I never get an answer that makes sense to me (if I get one at all, blank stares seem to be the most common answer which is amazing to me that people can do things so blindly, what ever happened to curiosity!?!? but that's a post for a different day).

So... Has anyone else heard of this? Is this related to a standard that I haven't been able to locate? Is this a "rule of thumb" in the industry that's just accepted and not necessarily based on anything? Some people say its based on AQL and/or probability theory, but can't seem to follow the creative math used to justify these answers.

If anyone has insights - please let me know.

Thanks!

#### Statistical Steven

##### Statistician
Staff member
Super Moderator
All of these rules (10% rule or 20% rule) are based on the thinking of sampling proportional to lot size. This was an audit strategy, but has lost popularity as statistical methods became the norm.

In general these "rules of thumb" do more hurt than help....

#### Bev D

##### Heretical Statistician
Staff member
Super Moderator
Steven is correct of course. A XX% sample plan is just mythology; it has zero basis in reality; it is just convenient.

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#### rpanciocco

Thanks! I always thought the method was odd and found it even more troubling that I couldn't get a solid justification for it from anyone. I guess my mother was right to teach me to "trust my gut"

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