“What were we thinking?” mused our two quality managers, Marsha Mallow and Maura Thesame, recalling their assumption that a process is in control if all the X-bars fall above the lower control limit and below the upper control limit. The two are capable of endless new learning.
What they had forgotten, of course, was the application of the runs test rule, which indicates that seven or more points above or below the centerline indicate an out-of-control situation as well. The chart above indicates 10 X-bars above --reflecting a process that is out of control.
After this dramatic new learning last month, the pair is ready this month to attack yet another chart pattern. In this one, there are no runs with averages (X-bars) but the range section appears as below:
Marsha promptly tells the operator that the process is out of control, with Maura Thesame echoing the observation and adding that the technician is to be congratulated for reducing variability on line #1, as indicated by the chart. When the technician asks what he should do, Marsha Mallow tells him to buy donuts for the line #1 team. Is this the right advice?
a) Yes. In spite of its contribution to weight gain and high cholesterol, buying donuts is exactly what should be done--and the only action that the technician should consider.
b) No. In fact, some action needs to be taken, and the two managers should have indicated this to the technician.
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