Z 1.4 Inspection Levels

Pharmaceutical sampling

Question

I am using a reduced switching rule and I don’t understand the meaning of the numbers in the first box. Total noncomforming less than limit number? What’s my limit number?Does production stability mean capability? Would I use 1.33?  The table has an arrow to reduced, so would I move to the next box?

Answer

The ANSI/ASQ Z1.4 standard has three inspection levels: normal, reduced and tightened inspection.  Initially you start at normal inspection, and can move to either tightened or reduced inspection depending on how lots are dispositioned.  Based on Figure 1 of the standard, the determination to move amongst the levels can be ascertained.  When you get to the reduced inspection level (Table II-C), you need to read the footnote (†).  It states “If the acceptance number has been exceeded, but the rejection number has not been reached, accept the lot, but reinstate normal inspection.”

A stable process or production is less about a capability index, and more about the control chart of the data showing a stable process.  In other words, the process is stable over time.

Steven Walfish

For more information about inspection, please view the resources found here.

Confidence Levels

 

Data review, data analysis, data migration

Question

I would like to confirm if ASQ Z1.4-2008 attribute tables are calculated based on 95% confidence level? I am using Table II-A, on page 11.

Answer

ANSI/ASQ Z1.4 tables are not technically calculated based on a 95% confidence level.  The technical definition of AQL is the quality level that is the worst tolerable process average when a continuing series of lots is submitted for acceptance sampling.  Some interpret it to mean if a lot has AQL percent defective or less, a lot would have a high probability of being accepted based on the sampling plan.  The standard does not specify the probability of acceptance explicitly.  The operating characteristic curve (OC Curve and the tables define the AQL as the percent defective that has a 95% probability of acceptance.  So though it is not a 95% confidence level, it is a 95% probability of acceptance.

Steven Walfish

For more information about AQL, please view the resources here.

Sampling Foils, Films, and Labels

Manufacturing, inspection, exclusions

Question

My question is about sampling aluminium foils, films used in packaging and sticker labels received in rolls which are wound around a core. I can decide to chose the number of rolls to sample from using the tables given in Z1.4, but how should I decide on the amount of stickers and aluminium foil and film to be sampled? I ask this question since it is practically impossible to sample from within a wound roll.
Answer

The ANSI Z1.4 and Z1.9 standards might be applicable when all units do not have the same probability of being selected.  Since you cannot sample units closer to the core, and defects would never be detected unless they occur at the end of the roll, I would recommend a different strategy, either using a vision system (100% inspection) or in process inspection.

If you want to use the standard, the sample size should be based on the number of samples, not the number of rolls.  For example, a roll with 5000 labels would be an N=5000 not N=1.

Steven Walfish

Learn more about visual inspection here.

ANSI Z1.4 Reduced Inspection

Question

If you have Ac=0 and Re=2 what do you do for 1? I have not used the reduced sampling before, so am curious what should be done in this instance.

Answer

If you review the footnotes for Table II-C of ANSI Z1.4, you will see that there is a note (†) that states: If the acceptance number has been exceeded , but the rejection number has not been reached, accept the lot, but reinstate normal inspection (see 10.1.4).  So in your case, with a single reject, you would accept and reinstate normal inspection.

Steven Walfish

DPPM Calculation

Chart, graph, sampling, plan, calculation, z1.4

Question

Recently, there is a debate in my organization about Defective Parts Per Million (DPPM) computation.
Camp 1 – DPPM = (No of parts rejected / No of parts inspected) * 1,000,000
Camp 2 – DPPM = (No of parts rejected / No of parts received) * 1,000,000
We perform sampling inspection based on AQL.
Camp 1 insists they are correct and likewise for Camp 2.  Which is correct or more appropriate to reflect supplier quality?

Answer

This is not an uncommon question. If you look at the standard, they define the % nonconforming as the number of parts nonconforming/number of parts inspected x 100. If you are looking at DPPM, instead of multiplying by 100, you put in 1,000,000. This means that by your definition, Camp 1 is correct. This is also what was intended by the creators of the sampling scheme.

Jim Bossert
Sr Performance Improvement Specialist
JPS Hospital
ASQ Fellow, CQE, CQA, CMQ/OE, CSSBB, CSSMBB
Fort Worth, TX

Zero Acceptance Number Sampling Plans and the FDA

Pharmaceutical sampling

Question

There has been some debate over using the MIL-STD-1916 acceptance sampling plan over the ANSI/ASQ Z1.4-2003 (R2013) sampling plans.  The opinion is that the ANSI/ASQ Z1.4-2003 (R2013) is outdated and no longer an acceptable method of determining a qualification sample plan and the MIL-STD-1916 should be used in place of ANSI/ASQ Z1.4-2003 (R2013). Do you have information around this debate over which sampling plans are acceptable by the FDA?

Answer

FDA does not (and can not) tell you what sampling plan is to be used.  The FDA requirement is that the plan be statistically valid.  As long as you follow the regulation, you are meeting FDA requirements.

In medical device manufacturing the key point is to have the plan accept on zero defectives.  This point is not FDA but legalese.  It is based on past lawsuits.  The plan “Zero Acceptance Number Sampling Plans” by Nicholas L. Squeglia (available from ASQ) has been widely adopted for this reason.

ANSI/ASQ Z1.4 in not outdated and continues to be widely used.  It is the American National Standard Institute (ANSI) version of MIL-STD-105 which the government discontinued maintaining, allowing ANSI to maintain it along with many, many other MIL-STD’s as a government cost reduction.

MIL-STD-1916 can be used but it is not widely used because of its difficulty and practical use.

James Werner

AQL Clarifications

Automotive inspection, TS 16949, IATF 16949

Question

I am confused about the values used for AQLs. For example in Table II-A the AQL values range from 0.010 to 1000. Where do these values come from and what do they mean?

The table states, “AQLs, in Percent Nonconforming Items and Nonconformities per 100 Items .” At first I thought the values were percentages, but how can you have more than 100, as in 100%, as the values go up to 1000? Also how can there be more than 100 nonconformities per 100 items, unless one part can have multiple nonconformities?

Just looking for clarification on the AQL numbers, what they mean, and how to interpret them.

Answer

Let’s start with the definition of Acceptable Quality Level (AQL).  From Z1.4, the AQL is the quality level that is the worst tolerable process average when a continuing series of lots is submitted for acceptance sampling.  Although individual lots with quality as bad as the AQL can be accepted with fairly high probability, the designation of an AQL does not suggest that this is necessarily a desirable quality level. The AQL is a parameter of the sampling scheme and should not be confused with a process average which describes the operating level of a manufacturing process. It is expected that the product quality level will be less than the AQL to avoid excessive non-accepted lots.

The columns with percentages greater than 100% should not be included in the standard, but remain as indication of how to interpret lots where the entire sample is defective.  It has some statistical relevance with use of the switching rules, but for the general practitioner, it should be ignored.

Hope this helps.

Steven Walfish

Z1.4 Sample Size

Pharmaceutical sampling

Question

I am trying to determine the sampling size using my ANSI/ASQ Z1.4 table and I wanted to get some clarification. If I am using Table II A and my Sample Size Code letter is D, what would be my sample size? If it falls on an arrow does it mean that I have to change to the next sample size based on where the arrow points?

Answers

From Charlie Cianfrani:

If you are using Z1.4, your sample size is selected based on your lot size.  You would pick the AQL you need based on the risk you are willing to take for the process average of percent defective.  It is important to understand what you are doing when using sampling plans, what they are and the protection you are trying to ensure. Thus, the important step is to determine the AQL. Then you select the sample size to provide the level of protection you are striving to ensure. It is more important to understand the theory behind the tables than to mechanically use the tables.

From Fred Schenkelberg:

Use the sample size where the arrow points. In the 2008 and 2013 versions it explains this in section 9.4, “When no sampling plan is available for a given combination of AQL and code letter, the tables direct the user to a different letter. The sample size to be used is given by the new code letter, not by the original letter.”

From Steven Walfish:

The standard sample size for Code Letter D from IIA is a sample size of 8.  But depending on your AQL, a sample size of 8 would be inappropriate, so the standard has arrows to delineate alternative sample sizes to reach the target AQL.  So, you sample size and accept/reject values are changed.  For example, at an AQL of 0.25, you would move down to a sample size of 50, with an accept/reject of 0/1.  If the lot size is less than 50, you would need to do 100% inspection.  In other words, there is no sampling plan that can give an AQL of 0.25 without a minimum sample size of 50.

From James Werner:

Yes.  When using Z1.4 two items need to be known, lot size and the AQL (Acceptance Quality Limit).  You use Table I – Sample size code letters to determine the Sample size code letter based on the Lot or batch size.  In the question below that was determined to be “D”.  Next step is to use Table II-A to find the sample size related to the sample size code letter – D and the AQL.  On Table II-A go across the table’s row for letter D until it intersect the given AQL column heading.  If an arrow is in that intersection point, follow the arrow then go back to the sample size code letter column to find the actual sample size (if a up/down arrow is in there then you choose).

Example 1.  Code letter is D (as in the question below).  Let’s say the AQL is 0.25.  Starting at code letter D, move across that row until you intersect at the AQL 0.25 column.  There’s a down arrow this row/column intersection.  Follow the arrow downward until the “Ac Re” reads ” 0 1″.  Staying on this row go back to the Sample size code letter column and find Code Letter H and Sample size = 50.  This means for the lot size with code letter D and with an AQL of 0.25 the sample size = 50 and accept the entire lot if no nonconformances were found else reject the entire lot if 1 or more nonconformance were found in the sample.

Example 2.  Let’s say the Sample size code letter was determine from Table I to be “F”.  Looking at Table II-A; If the AQL = 0.65, then the sample size would be 20 and the lot would be accepted zero nonconformance.  But if the AQL = 0.15 then the sample size would be 80.

ASQ/ANSI Z1.4 is available for purchase in PDF as well as hard copy.

Acceptance Sampling Inspection

Automotive inspection, TS 16949, IATF 16949

Question

We have an Acceptance sampling inspection in place where we use the ANSI/ASQ Z1.4 – 2013 standard under Normal Inspection, using General Inspection Level II to drive our samples size and accept, reject criteria. We do not uses switching rules as we have always found them too difficult to manage. I have two questions.

If I have one lot that fails Acceptance sampling and I am trying to bound the issue is it suitable to bound it to the one affected lot if the lot before and after pass or do I need to carry out additional sampling.

My second question is if I have a batch that passes acceptance sampling but at a subsequent downstream process a defect being inspected for by the upstream acceptance sampling inspection is found how do I determine if the lot is acceptable? Do I trust the acceptance sampling inspection or react?

Answer

The first question is not an uncommon one and actually it is a good practice to isolate the lot and do 100% inspection of it.  That way you can estimate the % defective and if another failure occurs in the next 5 lots, then increase the sampling until you have some confidence that the supplier has fixed the problem.  Once that confidence is restored, then you go back to what you inspected originally.

The second question, is one that you have to understand how well do you follow the acceptance sampling process?  If your alpha level is at 95%, 5% of the time, you can accept a bad batch as good. That is the pure definition of the alpha risk.  If this failure falls within the 5%, your process is working and while you sort through the lot, and notify the supplier, it is not something that you over react to.

I hope this helps.

Jim

James Bossert, PhD, MBB, CQA, CQE, CqM/OE
Sr Performance Improvement Consultant

Switching Rules

Manufacturing, inspection, exclusions

Question 

We are planning to implement ANSI/ASQ Z1.4-2003(R2013) sampling inspection plan with our Finish products which are currently 100% inspected by QC Inspectors.  I read  about the importance of the switching rules  on a continuing stream of lots and have the following  questions:
1.Is it acceptable to select a specific plan (tightened, normal or reduced ) and use it without the switching rules?
2.Are there any exceptions which allow us to use a specific plan without applying  the switching rules?

Answer

  1. You can use any plan without using the switching rules but it does run the risk of not meeting the alpha risk in the end. These plans were developed to be used as documented. A normal plan is generally used and the switching rules come in when the clearance number has been obtained.  Some processes may never switch.  If you choose a plan that is tightened or reduced to start with, you potentially will either spend too much on inspection (tightened) or risk having bad product go to the customer (reduced).  It is a business decision for you to make if your customer is not demanding it.  The switching rules are there to protect the producer when the product is running very well or it has problems.
  2. If your customer is not requiring a particular plan, you can use what you want. It is a business decision, no reason for any exceptions.

I hope this helps.

Jim Bossert
Sr Performance Improvement Specialist
JPS Hospital
ASQ Fellow, CQE, CQA, CMQ/OE, CSSBB, CSSMBB
Fort Worth, TX