Data Analysis with Microsoft Excel: Updated for Office 2007

Chapter 12 Quality Control 491

In contrast, the process depicted in Figure 12-2 is out of control. Both the

fourth and the twelfth observations lie outside of the control limits, leading

us to believe that their values are the result of uncontrolled variation. At

this point a shop manager, or the person responsible for the process, might

examine the conditions for those observations that resulted in such extreme

values. An analysis of the causes could lead to a better, more effi cient, and

more stable process.

Figure 12-2

A process

not in

control

process value

exceeding control

limits

process value

below control

limits

Figure 12-3
A process out
of control
because of
an upward
trend

Even control charts in which all points lie between the control limits

might suggest that a process is out of control. In particular, the existence of a

pattern in eight or more consecutive points indicates a process out of control,

because an obvious pattern violates the assumption of random variability. In

Figure 12-3, for example, the last eight observations depict a steady upward

trend. Even though all of the points lie within the control limits, you must

conclude that this process is out of control because of the evident trend the

data values exhibit.