Page 196 - The Handbook for Quality Management a Complete Guide to Operational Excellence

182 P r o c e s s C o n t r o l Q u a n t i f y i n g P r o c e s s Va r i a t i o n 183

47.5

42.5 UCL = 42.877

37.5

32.5
Defective 27.5 PCL = 27.933

22.5

17.5

12.5 LCL = 12.990
1 4 7 10 13 16 19 22 25 28
Figure 9.13 np chart example constructed using Green Belt XL software
(Courtesy of www.qualityamerica.com by permission).

the two control limits are set at plus and minus three standard devia-
tions. If the process is in statistical control then virtually all subgroup
occurrences-per-unit should be between the control limits and they
should fluctuate randomly about the centerline.

subgroup count of occurrences
u =
e
subgroup size in units
sum of subgroup occurrences
u =
sum of subgroup sizes in units
u
LCL = u − 3
n

u
UCL = u + 3
3
n
In the above equations, n is the subgroup size in units. If the subgroup
size varies, the control limits will also vary.
The data in Table 9.9 were obtained by opening randomly selected
crates from each shipment and counting the number of bruises on peaches.
There are 250 peaches per crate. Our unit size will be taken as one full crate;

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