Quality and Reliability Methods - SAS

Chapter 3 Introduction to Control Charts 55
What is a Control Chart?
What is a Control Chart?
A control chart is a graphical way to filter out routine variation in a process. Filtering out routine variation
helps manufacturers and other businesses determine whether a process is stable and predictable. If the
variation is more than routine, the process can be adjusted to create higher quality output at a lower cost.
All processes exhibit variation as the process is measured over time. There are two types of variation in
process measurements:
• Routine or common-cause variation. Even measurements from a stable process exhibit these random ups
and downs. When process measurements exhibit only common-cause variation, the measurements stay
within acceptable limits.
• Abnormal or special-cause variation. Examples of special-cause variation include a change in the process
mean, points above or below the control limits, or measurements that trend up or down. These changes
can be caused by factors such as a broken tool or machine, equipment degradation, and changes to raw
materials. A change or defect in the process is often identifiable by abnormal variation in the process
measurements.
Control charts quantify the routine variation in a process, so that special causes can be identified. One way
control charts filter out routine variation is by applying control limits. Control limits define the range of
process measurements for a process that is exhibiting only routine variation. Measurements between the
control limits indicate a stable and predictable process. Measurements outside the limits indicate a special
cause, and action should be taken to restore the process to a state of control.
Control chart performance is dependent on the sampling scheme used. The sampling plan should be
rational, that is, the subgroups are representative of the process. Rational subgrouping means that you will
sample from the process by picking subgroups in such a way that special causes are more likely to occur
between subgroups rather than within subgroups.
Parts of a Control Chart
A control chart is a plot of process measurements over time, with control limits added to help separate
routine and abnormal variation. Figure 3.2 describes the parts of a simple control chart.