Those variations are normal variations.
The exceptional variation would be when you decide to drive into work, again on a
weekday, that just happens to have a holiday fall on that day. Let??™s say that, due to a
crushing workload, you decide to drive into work on Labor Day (which for those of
you not familiar with the United States??™ holidays, always falls on the first Monday
of September). Hopefully, you do not always go to work on a holiday, so this is an
exception. You follow the same route, but it doesn??™t take as long, you don??™t hit as
many stoplights, and the number of cars in front of you is lessened.
Due to normal variation in any process, the numbers (in our example, the number
of cars waiting at the stoplight, the number of accidents that may occur) can
change when the process really has not. So, we need to understand both the numbers
relating to our processes and the changes that occur in our processes, so that we
may respond appropriately.
Other terms that you may see are common causes of variation and special causes
of variation, and common cause systems and special cause systems.
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