We can further combine them into the stacked
XmR chart shown in Figure 18.13. By stacking or aligning the individuals chart on
top of the moving range chart you can better compare the monthly data.
In the moving range chart at the bottom of Figure 18.13, the center line is
4.66. The upper limit is 15.21. In the individuals chart at the top of Figure 18.13,
the center line is 17.28. The upper limit is 29.67. The lower limit is 4.89. You can
clearly see cycles in the individuals chart. The line for December to January shows
a precipitous drop that appears to occur every year. The line for February begins a
climb back up. The line for March to April goes downward. The line for May goes
back up. June and July are down. July to December is up. These cycles also show up
in values for the moving range. Note the spikes for moving range values in January
UCL = 23.179
LCL = 9.154
CEN = 16.167
0
5
10
15
20
25
30
Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec
Figure 18.12 indi iduals chart for 2000.
252 n Interpreting the CMMI
Individuals Chart for 2000, 2001, 2002
UCL = 29.
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