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292 Part IV: Quality Assurance
Continued
Table 19.9
Number of nonconformities on printed boards for laptops per sample, with
varying sample size.
Part IV.B.4
Number of
Number of
nonconformities
nonconformities
Day Sample size per sample Day Sample size per sample
1 3 33 16 5 40
2 5 40 17 4 40
3 3 38 18 5 37
4 5 43 19 4 40
5 5 45 20 5 37
6 5 35 21 4 39
7 5 35 22 5 48
8 3 41 23 5 39
9 3 40 24 5 36
10 5 30 25 3 39
11 5 36 26 3 36
12 5 40 27 5 43
13 3 33 28 3 36
14 4 36 29 4 49
15 4 38 30 3 40
The control limits are calculated for each individual sample. Thus, for example, the control
limits for sample one are given by
UCL = u+ 3 u/ n1
= 9. 22+ 3 9. 22/ 3 = 14.
48
CL = u = 922 .
LCL = u 3 u/ n = 9. 22 3 9. 22/ 3 = 3. 96.
1
The control limits for the rest of the samples are calculated in the same manner. The
center line, however, remains the same. The u chart for the data in Table 19.9 is shown
in Figure 19.15.
The u chart in Figure 19.15 shows that all the points are within the control limits.
Moreover, points eight and nine, in particular, fall very close to the upper control
limits. All these observations about the process indicate that there may be some special
causes present in the process. That is, the process may be on the verge of being unstable.
Therefore, all precautions should be taken to avoid the process becoming unstable,
but certainly without overcontrolling it.
Continued