Table 12-1 Types of Variation
Variation Descriptive Remedy
Controlled Variation that is native to the
process, resulting from normal
factors called common causesRedesign the process to result in
a new set of controlled variations
with better properties.
Uncontrolled Variation that is the result of
special causes and need not be
inherent in the processAnalyze the process to locate
the source of the uncontrolled
variation and then remove or fi x
that special cause.Control Charts
The principal tool of SPC is the control chart. A control chart is a graph of the
process values plotted in time order. Figure 12-1 shows a sample control chart.Figure 12-1
A control
chart
upper control limit (UCL)center linelower control limit (LCL)process valuesThe chief features of the control chart are the lower and upper control
limits (LCL and UCL, respectively), which appear as dotted horizontal lines.
The solid line between the upper and lower control limits is the center line
and indicates the expected values of the process.
As the process goes forward, values are added to the control chart. As
long as the points remain between the lower and upper control limits, we
assume that the observed variation is controlled variation and that the pro-
cess is in control (there are a few exceptions to this rule, which we’ll dis-
cuss shortly). Figure 12-1 shows a process that is in control. It is important
to note that control limits do not represent specifi cation limits or maxi-
mum variation targets. Rather, control limits illustrate the limits of normal
controlled variation.490 Statistical Methods