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Re: Inspection Levels

From: Stan Hilliard
Date: 24 Apr 2002
Time: 00:36:15

Comments

In Z1.4, the special inspection levels S4 to S1 provide progressively smaller sample size. I think that is all that can be said about them in general.

To understand the effect of changing from one inspection level to another on the probability of accepting a bad lot (the consumer's risk=beta) or of rejecting a good lot (the producer's risk=alpha) -- you need to look at the oc curves. www.samplingplans.com/usingoccurves.htm

I usually will develop a sample size (n) and acceptance number (Ac) first to achieve the oc curve that gives the protection that I want. I use the software program TP105 for this. www.samplingplans.com/software_oc.htm

Then if I need to refer to a plan in Z1.4 for communication purposes I work Z1.4 backwards -- start with n & Ac and look up the sample size code letter, the AQL, and the inspection level. If there is no organizational necessity to refer to Z1.4 you can skip this step.

I think that the key point here is to use the probabilities of the oc curve as the basis to decide whether the sampling plan (n,Ac) is suitable for its intended application.


Last changed: November 20, 2007