bility can be changed only by changing one of the
process components that determine the process—people,
machines, material, methods, and environment.
While the above process was under statistical control,
it still incurred a variance of 17,200 ounces. This
variance indicated that the process was unable to meet
the standard allowed for raisin consumption and that
the only remedy was to attempt to improve the process
by changing one of the process components.
Processes can have measured capability and be
under statistical control but not produce to desired
specifications. But processes cannot be improved until
they are under statistical control. During the four
months when the process was not in statistical control,
its variation became erratic from day to day and week
to week.
Donald Wheeler, well known for his writing and
consulting experience with understanding variation,
has stated:
When a process displays a lack of statistical control the
pattern of variation will be inconsistent from day-to-day.
The variation in the process,and the variation in the
product,are said to be due to both common causes and
assignable[special] causes. Such a process will be unpredictable....
When a process displays a lack of statistical control
the control chart will detect the presence of the
assignable causes. Each and every signal on a control
chart presents an opportunity to gain more insight into the
process.4
The points at which the process is not in control represent
points in time where other quality tools such as
Pareto charts and cause-and-effect diagrams along with
the PDCA cycle may be employed in an effort to find a
solution to the problems and improve processes.
The auditor next asked that scales be purchased to
weigh the fruit on a sample basis prior to and after storage
and at the completion of the packaging process. A
sample of cereal boxes and fruit was emptied, and the
cereal and fruit were weighed separately. This weighing,
along with those when the fruit was received, were
charted using SPC. He also spent a great deal of time
observing the moving of fruit to and through the packaging
line. Through this analysis of both the fruit operation
and the cereal operation, he concluded that the
problem was with the fruit process, not the packaging
process.
After further examination, the auditor discovered
that the packaging machines had been adjusted so that
less fruit was being added to each box; however, the