A First Course in Quality Engineering, 2nd Edition

Quality in Production—Process Control I 247

plotted in Figure 4.9a in relation to the limits. The chart shows that the process

is not in-control. Action is necessary to find out why there are p

i

values outside

the limits. The reason was that several large p

i

values occurred at the start of the

process because the process was going through a set up. When the set up was

complete, the p

i

values settled down at a lower level.

When the causes for the values outside the limits are known and there is assur-

ance that these causes would not reoccur, the p

i

values outside the limits are

(a)

0.00

0 10

Sample number

20

LCL = 0

P = 0.01636

UCL = 0.07019

0.01 ...