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Part Three
Planning and control
Figure S17.5 The conventional and Taguchi views of the cost of variability
Why variability is a bad thing
Assignable variation is a signal that something has changed in the process which therefore
must be investigated. But normal variation is itself a problem because it masks any changes
in process behaviour. Figure S17.6 shows the performance of two processes both of which are
subjected to a change in their process behaviour at the same time. The process on the left has
such a wide natural variation that it is not immediately apparent that any change has taken
place. Eventually it will become apparent because the likelihood of process performance
violating the lower (in this case) control limit has increased, but this may take some time.
By contrast, the process on the right has a far narrower band of natural variation. Because
of this, the same change in average performance is more easily noticed (both visually and
statistically). So, the narrower the natural variation of a process, the more obvious are changes
in the behaviour of that process. And the more obvious are process changes, the easier it is to
understand how and why the process is behaving in a particular way. Accepting any variation
in any process is, to some degree, admitting to ignorance of how that process works.
Figure S17.6 Low process variation allows changes in process performance to be readily
detected