Results

point), it is appropriate to construct and analyze an X-bar chart. If the

X-bar chart shows evidence of an unstable process, the entire process is unstable. Hence, for process stability, both the R- and X-bar charts must show evidence of a stable process. Note that Figure 7.8 shows a stable process. Step 6: If the R-chart shows evidence of a stable (in control) process and if the corresponding X-bar chart reveals an unstable process, take the appropriate action to stabilize the X-bar chart. If the appropriate action implies removing one or more subgroup statistics that fall outside the control limits, it may be necessary to reconstruct the R-chart without including the specific subgroup statistics eliminated in the X-bar chart.

Figure 7.8 shows a stable process regarding the range (variations between subgroups). An X-bar chart should be drawn next to verify this information. If the R-chart had shown an unstable process, it would be appropriate to take action to prevent the recurrence of the special cause of the variation. After stabilizing the process, an X-bar chart should be constructed.

Figure 7.8 An R-chart showing waiting time for a specialist appointment (in days).