Looking for assignable causes
for processes that are running predictably is a waste of time because you won??™t find
any. Work instead on improving the process itself. When a process is unpredictable,
that means it is not operating consistently. It is a waste of time to try to improve
the process itself. In this case, you must find out why it is not operating predictably,
and detail the whys as specifically as possible. To do that, you must find and
fix the assignable cause(s); that is, the activity that is causing the process to behave
erratically.
An example of fixing an assignable or special cause of variation in our driving to
work example would be if your car breaks down on the way to work. If this happens
once, you might not take any action. However, if your car is old and breakdowns
occur frequently, you might decide to remove this special cause by buying a newer
car.
In contrast to predictability of a process, we may want to consider if a process
is capable of delivering what is needed by the customer. Capable processes perform
within the specification limits set by the customer.
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