Chapter 12 Quality Control 499on the shop fl oor by workers who have had little or no formal statistical train-
ing (and might not have ready access to Excel), the method for estimating s
is simplifi ed and the normal approximation is used to construct the control
chart. The difference is that when s is unknown, the control limits are esti-
mated using the average range of observations within a subgroup as the mea-
sure of the variability of the process. The control limits are
LCL 5 x 2 A 2 RUCL 5 x 1 A 2 RR represents the average of the subgroup ranges, and x is the average of the
subgroup averages. A 2 is a correction factor that is used in quality-control
charts. As you’ll see, there are many correction factors for different types of
control charts. Table 12-3 displays a list of common correction factors for
various subgroup sizes n.
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