How to work with Mil Std 105E OC curves?

Bev D

Heretical Statistician
Staff member
Super Moderator
#11
Dave - yes there are some segments of industry that have laws - I too have some parts of our product line under FDA control and I've worked in aerospace and defense. (ugh!)

However, this guy was doing what amounts to a 'dock audit' after a 100% inspection. And given the other things he could do that I listed - he could fully justify statistically, scientifically and even logically a relatively small sample that wasn't derived from traditional statistical plan approaches such as an AQL (an outdated useless but extrememly confusing system by the way) which was my meaning although sloppily articulated.

A traditional approach such as the AQL system is based on the assumption and intent that one is only sampling. And therefore one has to specify some defect level that one wants to have some specified confidence in detecting. If you really are looking to protect yourself from very low defect rates you will get a very large sample size. BUT - and this is the key but - he isnt only sampling he is doing is doign a sample after a 100% inspection. If he were to use the AQL he would have an uneccessarily large sample size.

There is a big difference between justifying a particular sample plan with statistics and using a statistically based sample plan generator like an AQL system: hence your own bolding: valid statistical rationale.
even if he were in a highly regulated legal environement if he wre to do the things I listed he woudl have a valid statistical rationale.
 
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Jim Wynne

Staff member
Admin
#12
There is a big difference between justifying a particular sample plan with statistics and using a statistically based sample plan generator like an AQL system: hence your own bolding: valid statistical rationale.
even if he were in a highly regulated legal environement if he wre to do the things I listed he woudl have a valid statistical rationale.
There you have the problem: valid statistical rationale. The problem isn't quite that simple though. Valid statistical rationale is extraordinarily unhelpful when expected to do magic, or when proper planning hasn't been done. Consider the following scene between a QA manager, whom I'll refer to as QAM, and a statistically astute person, whom I'll refer to as SAP :notme: SA:

QAM has x lots of material (where x is some unmanageably large number) that must be inspected today. They are waiting on his receiving or shipping dock, or in his inspection area, somewhere on the production floor, or some combination of all of those places. He finds himself in the position of being unable to defy the laws of physics. He can't be in more than one place at the same time, and he can't expand time to allow for what needs to be done. It's clear that he needs to do sampling inspection if deadlines are going to be met, but he's unsure how to select samples that will give him a reasonable level of confidence that something won't blow up in his face. He calls on SA for help, and they have a conversation:

SA: Let's look at this first lot. How many defectives do you think there might be in it?

QAM: I have no way of knowing. Maybe none, maybe a lot. It varies.

SA: OK, we'll get back to that. What sort of confidence level do you need?

QAM: I need to be very confident. I'll get my arse kicked if anything bad gets through.

SA: OK. You can't estimate the level of defects or the confidence level.

QAM: No. So how big should my sample size be?

SA: Let's not get ahead of ourselves...
(QAM looks impatiently at his watch, and rolls his eyes)
SA: There are an awful lot of parts here, and a lot of containers all stacked up. Assuming that I can help you determine a sample size, how will you make sure the sample is random?

QAM: Random? RANDOM??? Well, I guess I'll have to take some from each container.

SA: That's not random. "Random" means that every member of the population will have an equal chance of being selected.

QAM: It ain't gonna happen. Look around.

SA: Even if we had a reasonable defect level and confidence level, if your sample isn't random, all bets are off. There's no point in trying to use a statistically valid plan.

QAM: *&$# statisticians!

The moral of the story is that there isn't much point in doing "AQL" unless the rules for it are followed. Math is funny that way. But there is a two-word solution to the problem: Process Control.
 

Bev D

Heretical Statistician
Staff member
Super Moderator
#13
That too, Jim.
I've been in that space too many times.
fortunately I'm in a company now that when I say "OK, just inspect them all or flip a coin: heads the parts are all acceptable" they get it. (a lot of our tests are destructive.)
 
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