Comparison of Level I and Level II Sampling Plans in Z1.4
The following operating characteristic curves show the effect of changing from the
current General Inspection Level II sampling plans to General Inspection Level I
sampling plans.
The set of operating characteristic curves have an associated AQL value of 1% for
various lot sizes and sample size code letters
As you can see by comparing operating characteristic curves for Level I and II sampling
plans, there is no loss of statistical power for values of quality (%defective) in the
neighborhood of the AQL or better (as it should be since plans being compared both have
the same specified AQL). The only issue would be if the slight loss of statistical power
for quality levels worse than the AQL would justify staying with the larger sample size
Level II plans. When you look at the two operating characteristic curves being compare
for each case, you will note that each plan will accept good quality equally well and each
plan will reject poor (?) quality equally well (quality levels at the maximum RQL for
both plans). The only measurable differences exist for quality levels between the AQL
and RQL.
When you compare the operating characteristic curves for the Level I plan (sample size
code letter H) and the Level II plan (sample size code letter K) obtained for lot sizes 1201
to 3200 and an AQL of 1%, you will note that both plans afford comparable performance
for quality level in the neighborhood of the AQL. In addition, you will note that the two
operating characteristic curves track each other quite well for quality levels greater than
the AQL. For example, the Level I plan (sample size = 50) has a 50% probability of
accepting/rejecting if the quality level is 3.33% defective whereas the Level II plan
(sample size = 125) has a 50% probability of accepting/rejecting if the quality level is
2.94% defective. My opinion would be that a 150% increase in sampling cost (sample
size goes from 50 to 125) is not justified for this slight difference in protection for quality
levels 2.94% - 3.33% defective. Effective process controls activities are the way you will
insure the quality of your produce is much better than theAQL.
COMPARISON OF VARIOUS LEVEL I & II SAMPLING PLANS FOR AQL = 1%
AQL = 1%
LOT SIZE: 1.2K- 3.2K
LEVEL I AND II
SAMPLE SIZE CODE LETTER: H AND K
The equivalent operating characteristic curves for the other lot size code letters are shown
below for 1% AQL plans.
LOT SIZES 3.2K – 10K:
SAMPLE SIZE CODE LETTERS J(n=80, c<2) and L(n=200,c<5)
LOT SIZES 10K– 35K:
SAMPLE SIZE CODE LETTERS K(n=125, c<3) and M(n=315,c<7)
LOT SIZES 35K – 150K:
SAMPLE SIZE CODE LETTERS L(n=200, c<5) and N(n=500,c<10)