Decisions

Section 3 Control Charts

Lesson S03-07

Text: Section 3 pages 34 – 45

Duration: 22 minutes

Decisions

As in almost all decision making processes, the interpretation of control charts is gambling. In fact, any time a decision must be made, and the truth is not known with certainty, we are gambling.

First a brief discussion on sampling error, then a set of descriptions of signals that provide strong evidence of a special cause of a process change.

Points beyond the control limits, runs, trends, and cycles apply to all control charts discussed in this course. They do not depend on the statistic being plotted following any particular distribution.

Type I and Type II Errors

When sampling we are gambling the results will reflect the actual full dataset. It is possible that the data will mislead us, yet being aware of these errors permits us to minimize their impact.

The purpose of this section is to review the signals and patterns that should be identified on X̄ and R (or s) charts. The most important concept is that these signals and patterns provide strong evidence that the process is out of control. However, they do not provide a guarantee that the process is out of control. Thus, we should always be aware of the risks we take when we draw incorrect conclusions. Furthermore, understand that there is always a risk of making an erroneous decision, but we can minimize that risk.

Points Beyond the Control Limits

Even when a process is stable, it is possible to see points beyond control limits. This event has already been alluded to, and it occurs 27/10,000 times. However, points beyond control limits are much more likely to come from an unstable process than a stable one. Hence, we are better off concluding that the process has changed.