Attribute Data - What is the required sample size (medical field)?

C

caveman123

#1
Just curious to see what rationale people typically use to based their sample size on for attribute data? For the company I work for our procedure gets very involved for variable data, but for attribute there seems to be a disconnect (from statistic backing).

Is there a good approach to justify a sample size that is feasible (no 200, 300 pieces inspection) yet satisfies the statistic requirement?
 
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N

Nancy Collins

#3
Re: Attribute Data - What is the required sample size

I am trying to resolve the same questions myself; I would love to see further discussion on this. Many articles recommend the binomial direct calculation or the normal approximation to the binomial without describing the assumptions. Upon investigation, my process(es) do not meet the assumptions. When using AQL as a required performance level to choose the lot tolerance percent defective at which to evaluate, I get sample sizes that are larger than my lot size or close to lot sizes, but the binomial requires the sample to be a) small when compared to lot size and b) to be from a stream of production. Many of the processes under evaluation can be considered streams, but some are not; they are manufactured in the same sized lots and then may not be made again for weeks.

I think, because the desired LTPD or 'reject quality level' is so small, by desire and nature of the product and history (prior to validation of the particular processes that are within many in the manufacture of the product), that the Poisson approximation to the binomial should be used, but the quantities turn out to be very similar to the binomial.

Anybody else resolved these issues? Dodge-Romig tables based upon lot sizes and LTPD?

Thanks very much.
 
B

brahmaiah

#4
Re: Attribute Data - What is the required sample size?

Just curious to see what rationale people typically use to based their sample size on for attribute data? For the company I work for our procedure gets very involved for variable data, but for attribute there seems to be a disconnect (from statistic backing).

Is there a good approach to justify a sample size that is feasible (no 200, 300 pieces inspection) yet satisfies the statistic requirement?
The most ideal method to inspect attribute data for a lot quantity of 200-300 nos. is 100% inspection.But if you are keen on sampling you may choose'0' AQL and follow sampling tables.In that case the possibilities of accepting your lot is there only if the process Cpk is >1.33.
If the part is not critical you may choose a higher AQL and go ahead with sampling plans.(But higher AQL is not acceptable in TS16949.)
V.J.Brahmaiah
 
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Stijloor

Staff member
Super Moderator
#5
Re: Attribute Data - What is the required sample size?

The most ideal method to inspect attribute data for a lot quantity of 200-300 nos. is 100% inspection.But if you are keen on sampling you may choose'0' AQL and follow sampling tables.In that case the possibilities of accepting your lot is there only if the process Cpk is >1.33.
If the part is not critical you may choose a higher AQL and go ahead with sampling plans.(But higher AQL is not acceptable in TS16949.)
V.J.Brahmaiah
That's incorrect!

From ISO/TS 16949:

7.1.2 Acceptance criteria

Acceptance criteria shall be defined by the organization and, where required, approved by the customer.

For attribute data sampling, the acceptance level shall be zero defects (see 8.2.3.1).
Emphasis mine.

Stijloor.
 
N

Nancy Collins

#6
Re: Attribute Data - What is the required sample size

I should add that I am trying to establish sampling and sample sizes for estimating process yield based on attribute data for Process Validations.
 
C

caveman123

#7
Re: Attribute Data - What is the required sample size

I should add that I am trying to establish sampling and sample sizes for estimating process yield based on attribute data for Process Validations.


I am on a similar page. Not so much of attribute data for production lot acceptance, rather how to handle the acceptable sample size for Design Verification.

Sorry that wasn't clear before:)
 

Marc

Hunkered Down for the Duration with a Mask on...
Staff member
Admin
#8
Re: Attribute Data - What is the required sample size?

That's incorrect!

From ISO/TS 16949: <snip>
Note that no one mentioned TS 16949 until your post, and the thread was originally started in the ISO 13485 forum, so TS 16949 requirements may not be applicable.
 

Bev D

Heretical Statistician
Staff member
Super Moderator
#10
If I'm readign the questions correctly and the posters with questions are concerned about sample size for verification adn validation (or IQ, OQ, PQ)

We have had a few of these questions in the forum. and these situations are absolutely not addressable with AQLs, RQLs, mil-stds etc because this is not acceptance sampling.

Unfortunately the answer is more complex than can be handled with a general question. specific situations are much better to tease out the important subtleties...

The one general comment that can be made is that this situation will use sample size calcualtions that one would do for hypothesis and DOE type testing/experiments.

If we can get a few specific questions, We can help with that. Of course if I've missed teh mark and the questions really are related acceptance sampling of completed lots, let us know!
 
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