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You are here: Home / Quality Control Tips / Inspection level vs. inspection severity

Inspection level vs. inspection severity

April 1, 2011

Inspection level vs. inspection severity

When I started doing QC inspections, my first client’s quality manager was interchangeably mentioning “level II” and “normal level”. And same thing with “level I” and “reduced level”.

I purchased the relevant standard and I read it sideways (it is quite boring). I never picked on the difference between these concepts. But this week someone at work told me I was wrong about it. I searched the internet and I found this comment from Tim Folkerts on the Elsmar Cove forum:

The nomenclature seems to confuse people. Level II is considered standard, but it is not the same as “normal inspection”. Level I reduces the sample size, but it is not the same as “reduced inspection.” Level II Reduced Inspection is roughly (but certainly not exactly) the same as Level I Normal Inspection. And so on….

That opened my eyes on the difference between the “inspection level” (which is very often used by QC firms and their clients to adjust the number of samples to check) and the “inspection severity” (which is supposed, in theory, to be set according to precise switching rules).

What the standard says about inspection level:

The inspection level designates the relative amount of inspection. Three inspection levels, I, II and III, are given in Table 1 for general use. Unless otherwise specified, level II shall be used. Level I may be used when less discrimination is needed or level III when greater discrimination is required.

Four additional special levels, S-1, S-2, S-3 and S-4 are also given in Table 1 and may be used where relatively small sample sizes are necessary and larger sampling risks can be tolerated.

What the standard says about inspection severity:Quality Control Tips

Normal inspection: use of a sampling plan with an acceptance criterion that has been devised to secure the producer a high probability of acceptance when the process average of the lot is better than the acceptance quality limit.

NOTE Normal inspection is used when there is no reason to suspect that the process average differs from an acceptable level.

Tightened inspection: use of a sampling plan with an acceptance criterion that is tighter than that for the corresponding plan for normal inspection.

NOTE Tightened inspection is invoked when the inspection results of a predetermined number of consecutive lots indicate that the process average might be poorer than the AQL.

Reduced inspection: use of a sampling plan with a sample size that is smaller than that for the corresponding plan for normal inspection and with an acceptance criterion that is comparable to that for the corresponding plan for normal inspection

NOTE 1 The discriminatory ability under reduced inspection is less than under normal inspection.

NOTE 2 Reduced inspection may be invoked when the inspection results of a predetermined number of consecutive lots

Now you know… Semantic differences are important!

Filed Under: Quality Control Tips

Comments

  1. Guilhem says

    April 1, 2011 at 8:26 PM

    Ok I better understand too, nice follow up of your previous entry!

  2. Renaud Anjoran says

    April 1, 2011 at 8:36 PM

    Thanks Guilhem…

  3. maria says

    April 29, 2012 at 9:41 AM

    Hello.
    I just want to ask, Is it ok that we only use 1 inspection level in incoming inspection. Switching rules is not usually done. Please advise…

    • Renaud Anjoran says

      April 29, 2012 at 10:06 AM

      Yes, that’s what most people do. Choose a level that makes sense, and then stick to it.

    • Renaud Anjoran says

      July 7, 2012 at 11:33 AM

      You don’t have to use the switching rules, if you don’t want to…

  4. Piotr says

    July 5, 2012 at 6:01 PM

    Normal, Tighten and Reduced and levels I, II and III are equivalent, according to original procedure… If you look at the tables, the one corresponding to level II is called “Single sampling plan for NORMAL INSPECTION  (II.A)”, level I|I – “(…) tightened inspection (II.B)” and so with reduced and level I.

    • Renaud Anjoran says

      July 6, 2012 at 10:40 AM

      What “original procedure” do you refer to? In the tables shown in the latest standard, the severity is clearly different from the level (we can have level II for normal or reduced or tightened severity, etc.).

      • Piotr says

        July 7, 2012 at 1:53 AM

         Hi, I’ve got MIL-STD-105E, 10 May 1989 so it seems quite old, but anyway…

        • Piotr says

          July 7, 2012 at 2:23 AM

           I just checked MIL-STD-1916, 1 April 1996, which superseded 105E. Word “severity” doesn’t exist at all but… you’re right that now “Normal, Tightened and Reduced” as referred as “stages”, defined unclearly, while verification levels are something else, but not I-III and S1-4 anymore, but I – VII with totally different Code letters table. So confusing. Good I’m not using it for professional purposes 😉

          • Renaud Anjoran says

            July 7, 2012 at 11:32 AM

            Yes, it can be quite confusing. And this militari standard was replaced (and slightly adapted) by commercial institutions like ISO, ANSI…


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