Quality and Reliability Methods - SAS

Chapter 2 Statistical Control Charts 27
Statistical Quality Control with Control Charts
Statistical Quality Control with Control Charts
Control charts are a graphical and analytic tool for deciding whether a process is in a state of statistical
control and for monitoring an in-control process.
Control charts have the following characteristics:
• Each point represents a summary statistic computed from a subgroup sample of measurements of a
quality characteristic.
Subgroups should be chosen rationally, that is, they should be chosen to maximize the probability of
seeing a true process signal between subgroups.
• The vertical axis of a control chart is scaled in the same units as the summary statistic.
• The horizontal axis of a control chart identifies the subgroup samples.
• The center line on a Shewhart control chart indicates the average (expected) value of the summary
statistic when the process is in statistical control.
• The upper and lower control limits, labeled UCL and LCL, give the range of variation to be expected in
the summary statistic when the process is in statistical control.
• A point outside the control limits (or the V-mask of a CUSUM chart) signals the presence of a special
cause of variation.
• Analyze > Quality And Process > Control Chart subcommands create control charts that can be
updated dynamically as samples are received and recorded or added to the data table.
Figure 2.2 Description of a Control Chart
Out of Control Point
Measurement
Axis
UCL
Center Line
LCL
Subgroup Sample Axis
The Control Chart Launch Dialog
When you select a Control Chart from the Analyze > Quality And Process > Control Chart, you see a
Control Chart Launch dialog similar to the one in Figure 2.3. (The exact controls vary depending on the
type of chart you choose.) Initially, the dialog shows three kinds of information:
• Process information, for measurement variable selection