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:

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!