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Chapter 19: B. Statistical Process Control 279
Continued
Table 19.5
Number of nonconforming computer chips out of 1000 inspected each day
during the study period of 30 days.
Number of Sample fraction Number of Sample fraction
nonconforming nonconforming nonconforming nonconforming
Day x p i Day x p i
1 9 0.009 16 12 0.012
2 5 0.005 17 5 0.005
3 6 0.006 18 6 0.006
4 11 0.011 19 12 0.012
5 11 0.011 20 10 0.010
6 12 0.012 21 6 0.006
7 7 0.007 22 7 0.007
8 11 0.011 23 11 0.011
9 6 0.006 24 11 0.011
10 6 0.006 25 9 0.009
11 8 0.008 26 5 0.005
12 5 0.005 27 12 0.012
13 8 0.008 28 11 0.011
14 5 0.005 29 7 0.007
15 8 0.008 30 9 0.009
First we calculate the sample fraction nonconforming values (p i ), which are listed
in columns three and six of Table 19.5. Substituting the sample fraction nonconforming
values in equation (19.31), we get
p – = 0.00837.
Plugging the value of p – = 0.00837 and n = 1000 into equations (19.32) and (19.34) we get
the control limits for the p chart, that is
UCL = 0.01701
CL = 0.00837
LCL = 0.0
The p control chart for the data in Table 19.5 is shown in Figure 19.10. From the control
chart in Figure 19.10 we observe that all the points are well within the control limits. We
should note, however, that starting from point number nine, seven successive points
fall below the center line. This indicates that from day nine through 15, the number
of nonconforming chips was relatively low. Investigation to determine the process
Part IV.B.4
Continued