Inspection Level Interpretation - ANSI Z1.4-2008

P

Petemxa

#1
Hello,

I have a question about how to interpret the "Inspection Level" as described in ANSI Z1.4-2008.

ANSI Z1.4-2008 describes General Inspection II as the standard inspection level, however you can use special inspections S-1 though S-4 when larger sampling risk can or must be tolerated.

Is there anyway to quantify this risk? How much more risk am I taking by choosing an S-2 instead of S-4 or S-2 instead of G-II?

Thanks,
Peter
 
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howste

Thaumaturge
Super Moderator
#2
Take a look at the OC curves. They will show you the probability of accepting a lot with a given % nonconforming product.
 
K

kabus - 2011

#3
I apologize for bursting into your conversation, but I am also working with a sampling plan using special level S-3.
I compared the OC curves and, correct me if I'm wrong, but the comparison showed that for an AQL of 0.65% lot sieze of 22,000 units...using S-3 inspection level a lot with 0.256 non-conforming units could be accepted 95% of the time while using the inspection level II this probability increases to 0.830 with an increase in the sample size as well from N=20 to N=315. Is this correct?

I am having trouble interpreting the tables. Could you please explain the relationship between the Pa (99.0, 95.0, 90.0 percentages on the left) and the nonconforming percentages in the center of the table under each AQL. Are the nonconforming percentages equal to the consumer risk? Also, what information do tables XII provide (these are the ones that reference Pa as 5% and 10%)?

Pardon my ignorance but I am trying to learn about sampling plans since this task was recently assigned to me. Anything will be helpful!!!
 
K

kabus - 2011

#4
Following the same line, that is development of sampling plans, I am supposed to determine a sampling plan for a gel product filled in tubes. There are some tests that are destructive; therefore I decided to go with an AQL of 0.65% and S-3 level of inspection for defects related to the product such as apperance, contamination and leakage. The lot size could be anywhere between 22,000 to 30,000 tubes. We want to keep the sample size small. Is this approach correct?

What factors shall be considered when determining the inspection level? The norm is level II but can a special level be used for in-process checks?

Is there a rule of thumb to develop sampling plans? or shall each be customized?

Thanks in advance for your help...reading the threads have helped me a lot...but I am still a roookie
 
P

Petemxa

#5
Howste,

Can you help me understand how to interpret the OC curves so I can get a quantifiable assessment of the risk I am adding to my inspection process from shifting my Inspection Level from a General II to a Special Inspection S-2.

The OC curve gives me the probability of incorrectly dispositioning a recieved lot for a defined Lot Size letter code using General Inspection II. I don't see how I can connect the probability they show for General Inspection II to the probability for that same lot size for Special Inspection II.

So for a Lot Size of 151-280 I get a Letter code of G which calls for a sample size of 32 pieces. Looking at the OC curve for Letter Code G, with an expected lot acceptance level of 99% and an AQL of 4.0, I get a 2.67% probability that I will incorrectly disposition the lot. This is using an Inspection Level II.

Now, if I want to use and inspection level S-2 for the same Lot Size of 151-280 I get a letter code of B, using AQL 4.0, which calls for a lot size of 3 pieces. How do I quantify the risk. If I look at the OC curve for letter code B and using the same 99% lot acceptance level and AQL of 4.0. I get a 0.33% probability that I will incorrectly disposition the lot. Now obviously this is incorrect. I can't improve my confidence by decreasing my sample size.

The OC curves are there to be used in reference to the normal Inspection Level II. Is there a way I can connect these OC curves to an inspection level S-1 through S-4?

Can I take the probability of letter code B of 0.33% and multiply it by 280/15? '280' being the max lot size for letter code G and '15' being the max lot size of letter code B. So I basically can I say: "If I have a .33% probability with a lot size of 15 I have a 6.16% probability with a lot size of 280, because I am not increasing the sampling size but the lot size increased by a factor of 18.667." That seems too simplified, but is it a good estimate?

Thanks,
Petemxa
 
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