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Chapter 19: B. Statistical Process Control 255
5. Minimize unnecessary variation. Unnecessary external causes of variation
should be reduced before the study begins. This includes overcontrolling
the process or avoiding obvious problems that could and should be
corrected even without the use of control charts.
6. Customer’s needs. This includes both any subsequent processes that use
the product or service as an input, and the final end item customer. For
example, any computer manufacturing company is a customer of the
semiconductor industry, a car manufacturing company is a customer
of tire manufacturing companies, and in a paper mill the papermaking
unit is a customer of the pulpmaking unit.
Note that in all cases, a process log should be kept. It should include all relevant
events (big or small) such as procedural changes, new raw materials, or change of
operators. This will aid in subsequent problem analysis.
Benefits of Control Charts
Properly used, control charts can:
1. Be used by operators for ongoing control of a process
2. Help the process perform consistently and predictably
3. Allow the process to achieve higher quality, higher effective capacity
(since there will be either no or fewer rejections), and hence lower
cost per unit
4. Provide a common language for discussing process performance
5. Help distinguish special causes from common causes of variability
and hence serve as a guide for management to take local action
or action on the system
Rational Subgroups for Control Charts
It is very important to note that the rational subgroup or sample used to prepare a
control chart should represent subgroups of output that are as homogeneous as
possible. In other words, the subgroups should be such that if special causes are
present, they will show up in differences between the subgroups rather than in
differences between the members of a subgroup. A natural subgroup, for example,
would be the output of a given shift. It is not correct to take the product for an arbitrarily
selected period of time as a subgroup, especially if it overlaps two or more
shifts. This is because if a sample comes from two or more shifts then any difference
between the shifts will be averaged out and consequently the plotted point
won’t indicate the presence of any special cause due to shifts. As another example,
if the process used six machines it would be better to take a separate sample from
the output of each machine than to have samples each consisting of items from
all six machines. This is due to the fact that the difference between machines may
be the special cause of variation. It will be hard to detect this special cause if the
samples are not taken from individual machines. Thus, it is true to say that careful
Part IV.B