Guidance on Z1.4 Levels

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Q: My company is using ANSI/ASQ Z1.4-2008 Sampling Procedures and Tables for Inspection by Attributes, and we need some clarification on the levels and the sampling plans.

We are specifically looking at Acceptable Quality Limits (AQLs) 1.5, 2.5, 4.0, and 6.5 for post manufacturing of apparel, footwear, home products, and jewelry.

Do you have any guidelines to determine when and where to use levels I, II, and III? I understand that level II is the norm and used most of the time. However, we are not clear on levels I and III versus normal, tightened, and reduced.

Are there any recommended guidelines that correlate between levels I, II, III and single sampling plans, normal, tightened, and reduced?

The tables referenced in the standard show single sampling plans for normal, tightened, and reduced, can you confirm that these are for level II (pages 11, 12, 13)?

Do you have any tables showing the levels I and III for normal, tightened, and reduced?

A: Level I is used when you need less discrimination or when you are not as critical on the acceptance criteria. This is usually used for cosmetic defects where you may have color differences, but it is not noticeable in a single unit. Level III is used when you want to be very picky.  This is a more difficult level to get acceptance with, so it needs to be used sparingly or it can cost you a lot of money.

Each level has a normal, tightened and reduced scheme.  I am not sure about what you are asking for with respect to correlation to levels I, II and III and normal, tightened and reduced.  The goal is to simply inspect the minimum amount to get an accept or reject decision. Since inspection costs money, we do not want to do too much. Likewise, we do not want to reject much since that also costs money both in product availability and extra shipping.

Yes, the tables on pages 11, 12 and 13 are for normal, tightened, and reduced, but if you look at the letters for sample size, you will note that in most cases there are different letters for the levels I, II, and III.  Accept and reject numbers are based on the defect level and the sample size. The switching rules tell you when you can switch to either a reduced or tightened level. The tables can handle not just the levels I, II , and III, but also the special levels.

Jim Bossert
SVP Process Design Manger, Process Optimization
Bank of America
Fort Worth, TX

3 thoughts on “Guidance on Z1.4 Levels”

  1. There are (at least) four different things one needs to consider when looking at Z1.4 plans: Level, AQL, S/D/M, and N/R/T.

    Level, in some sense, controls the operating characteristic (OC) curve. As you state, the default is Level II. If you look at Table I you will see that the Level controls the Code Letter. If you look in the back, you will see the OC curves for each code letter. For example, with a sample size of 75 you will get a different code letter for each Level. In general, higher the code letters will lead to larger sample sizes.

    You need to determine the AQL. The AQL, for sampling, is the minimum acceptable process level. It does not allow a process to knowingly produce nonconforming material. Instead, it is a parameter of the sampling plan.

    For S/D/M you decide to one sample, two samples, or multiple (up to seven) samples. In a single sampling case you make the decision (accept/reject) after one sample. In a double sampling case, you may need two samples before you can make a decision, however, in most cases you will not need the second sample. All other things being equal, a double sample is the best choice because the expected number of items sampled will be smaller in the long run.

    For N/R/T, the sampling plans adjust themselves based on lot history. If you have a good quality history, you can get to reduced, which will lower cost even more. An item that switches to Tightened is a warning that the process is not performing well.

    Jim is not quite correct when he says the N/R/T goes with the Level. N/R/T goes with S/D/M. By this I mean that if you pick double sampling and have a poor history (2 of 5 lots rejected), then you go the Tightened plans for double sampling. You Level (and code Letter don’t change).


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