So, they may suggest moving the limits
to plus or minus some value. No! The only way to change the limits is to change the
process itself. Remember, the charts show the voice of the process. So if management
desires to improve the limits, they must improve the process. The numbers
shown in the charts reflect results of the process. Our job is to find what caused
these results. When analyzing the causes, and adjusting the process, remember that
the inputs and outputs of the process may be affected, as well as any other processes
that are dependent upon the original process under review. So, focusing on one or
two causes will have downstream ramifications and will probably affect parts of
your process, as well as other processes.
We have also seen the limits for the XmR charts calculated using median ranges
instead of average ranges. The median moving range is often more sensitive to
assigned causes when the values used contain some very high range values that
inflate the average. Remember, the median range is that range of numbers that
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254 n Interpreting the CMMI
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