AQL and Acceptance Plan Clarification

  • Thread starter Plastic Quality
  • Start date
P

Plastic Quality

#1
I am starting to look at acceptance plans and am running into some questions. Help would be appreciated:
1. Why are there c=0 sampling plans as well as sampling plans that allow for the detection of errors, c>0? I am under the impression that either will get you to the same AQL in the end.
2. It seems after reading different posts that
Zero Acceptance Number Sampling Plans by Squeglia is quite recommended. However, I am under the impression that this is for attribute data. My company doesn't use much attribute, but mostly variable data. What method should we use?
3. I have also been reading about MIL STD 105E (something that our company does use, barely- I am trying to change that) and that seems to be obsolete, again, what should my company use instead?
4. (this might be the long one) When using either a c=0 sampling plan or a c>0 plan I have come up with the question, what lot size should I use? We make small injection molded plastic parts with a fast cycle time. I can make 550+ parts per hour. When I look at the sample size for a lot, should I look at per hour, per 8 hour shift, per 24 hour day (we have lights out manufacturing), per month, or per year? We typically inspect parts every 8 hours as an in process inspection. As the lot size increases the amount we need to sample for every 8 hours goes down. I.E. for every 8 hours at 1%AQL I need 50 parts per 8 hours, but if I look at 24 hours of run I need 60 parts across 24 hours, or 20 parts for each inspection. This is HUGE for our small company. If I carry it out further and we estimate we make 1e6 parts per year, then we only need to inspect ~6 parts per inspection period. Where do I set my sample size?

Thank you for your input, I have appreciated the advice found on other threads on this website and am impressed with the knowledge found here on this site.


 
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Bev D

Heretical Statistician
Staff member
Super Moderator
#2
I'm presed for time right now but can handle the easy ones...

1. Why are there c=0 sampling plans as well as sampling plans that allow for the detection of errors, c>0? I am under the impression that either will get you to the same AQL in the end.
Yes they will BUT they will get you to different rejection levels. a c>0 plan is more discriminating. A recent fad based on misunderstanding statistics has resulted in soem customers and agencies rejecting c>0 plans based since they can't stand it that the lot can be accepted even tho there were actually defects in the lot...even tho an AQL clearly states that some level of defects are allowed in the lot...

2. It seems after reading different posts that [/SIZE]Zero Acceptance Number Sampling Plans by Squeglia is quite recommended. However, I am under the impression that this is for attribute data. My company doesn't use much attribute, but mostly variable data. What method should we use?

If you are using variable data ANSI Z1.9 is the common standard, but you can find other books on it as well.

3. I have also been reading about MIL STD 105E (something that our company does use, barely- I am trying to change that) and that seems to be obsolete, again, what should my company use instead?
Mil-Std 105 was replaced by ANSI Z1.4. It is for sampling by attributes so how do you reconcile that with question 2 where you state that you don't use attributes much? (by the way most companies measure in variables data and simply 'convert' to attributes by assigning the variables results as either 'pass' or 'fail'.)
 
Last edited:
P

Plastic Quality

#3
Mil-Std 105 was replaced by ANSI Z1.4. It si for sampling by attributes so how do you reconile that with question 2 where you state tha tyou dont' use attributes much?
That would be a misuse of Mil-Std 105 on our part. I will address that. Thank you for your assistance.
 

mdurivage

Quite Involved in Discussions
#4
I reccomend purchasing the book The Handbook of Applied Acceptance Sampling. It pretty much has all the info you will need and more!
 

Tim Folkerts

Super Moderator
#6

4. (this might be the long one) When using either a c=0 sampling plan or a c>0 plan I have come up with the question, what lot size should I use? We make small injection molded plastic parts with a fast cycle time. I can make 550+ parts per hour. When I look at the sample size for a lot, should I look at per hour, per 8 hour shift, per 24 hour day (we have lights out manufacturing), per month, or per year? We typically inspect parts every 8 hours as an in process inspection. As the lot size increases the amount we need to sample for every 8 hours goes down. I.E. for every 8 hours at 1%AQL I need 50 parts per 8 hours, but if I look at 24 hours of run I need 60 parts across 24 hours, or 20 parts for each inspection. This is HUGE for our small company. If I carry it out further and we estimate we make 1e6 parts per year, then we only need to inspect ~6 parts per inspection period. Where do I set my sample size?

As with any sampling decision, it comes down ultimately to economics. If you take one BIG lot, then the total number of items to test goes down (saving money). However, the potential for a BIG problem increases (potentially costing a lot of money). If you use each shift as a lot, then you at most have to reject (or sort) that shift's output when the sampling shows a problem. If you treat the entire year as a lot, and if the sampling shows a problem, you have to deal with rejecting (or sorting) the entire year's production!

Tim F
 
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