performance. Plant management agrees with this observation
and directs that, in addition to implementing the control chart
program, the engineering staff should analyze the process in an
effort to improve the process yield. This study indicates that
several adjustments can be made on the machine that should
improve its performance.
During the next three shifts following the machine adjustments
and the introduction of the control chart, an additional
24 samples of n = 50 observations each are collected. These
data are shown in Table 7.2, and the sample fractions nonconforming
are plotted on the control chart in Fig. 7.3.
From an examination of Fig. 7.3, our immediate impression
is that the process is now operating at a new quality
level that is substantially better than the center line level
of . One point, that from sample 41, is below
the lower control limit. No assignable cause for this out-ofcontrol
signal can be determined. The only logical reasons
for this ostensible change in process performance are the
machine adjustments made by the engineering staff and,
possibly, the operators themselves. It is not unusual to find
that process performance improves following the introduction
of formal statistical process-control procedures, often
because the operators are more aware of process quality and
because the control chart provides a continuing visual display
of process performance.
We may formally test the hypothesis that the process fraction
nonconforming in this current three-shift period differs