Page 73 - DMGT206_PRODUCTION_AND_OPERATIONS_MANAGEMENT

Production and Operations Management

Notes However, the process shown in the right-hand side figure is out of statistical control. Notice that
a single point is outside the control limits (above them). This means that a source of special cause
variation is present. A special cause variation is a variation that can be attributed to defects in the
process, i.e., the process is not in control as it should be. Having a point outside the control limits
is the most easily detectable out-of-control condition. Though the likelihood of this happening
by chance is only about 1 in 1,000, this source of special variation has to be isolated and dealt
with.

Figure 4.2: A Typical Cycle in SPC

Figure 4.2 illustrates the typical cycle in SPC. In the first segment, the process is highly variable
and out of statistical control. As special causes of variation are found and eliminated, the process
comes into statistical control in the second segment. Finally, through process improvement,
variation is reduced. The narrowing of the control limits generally is an indication of process
improvement. The chart in the figure is educative in that it shows that by eliminating special
cause variation the process can be brought in control; and by process improvement we can
reduce the process variation and move the control limits in towards the centerline of the process.
By following this system on an ongoing basis, we help to prevent the production of defectives.
The first step for the analysis of a control chart is to determine, what patterns one should look
out for? What are the patterns that indicate that a process is going out of control?

4.2.3 Analysis of Patterns on Control Charts

A control chart may indicate an out-of-control condition either when one or more points fall
beyond the control limits. This pattern may indicate an out-of-control condition which is due to
a special cause of variance from a material, equipment, method, or measurement system change.

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Caution It should be remembered that Control chart may also indicate wrong measurement
of a part or parts, miscalculated or wrongly plotted data points, and/or miscalculated or
wrongly plotted control limits.
Other patterns that should create concern are when the plotted points exhibit some non-random
pattern of behaviour. These situations are described below and need to be looked into:

1. A run of eight points on one side of the center line. This pattern indicates a shift in the
process output from changes in the equipment, methods, or materials or a shift in the
measurement system.
2. An unusual or non-random pattern in the data; e.g., a trend of seven points in a row
upward or downward. This may show a gradual deterioration or wear in equipment;
improvement or deterioration in technique.

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