vdoc

Chapter 19: B. Statistical Process Control 259
difference between two items is called the variation. Such variation occurs due to
two causes:
1. Common causes, or random causes
2. Special causes, or assignable causes
To better understand these causes, it is also very important to understand a process
and to have a concrete plan for future actions that would be needed to improve
the process.
Process Evaluation
Process evaluation includes not only the study of the final product, but also the
study of all the intermediate steps or outputs that describe the actual operating state
of the process. For example, in the paper production process wood chips and pulp
may be considered intermediate outputs. If the data on process evaluation are
collected, analyzed, and interpreted correctly, they can show where and when
a corrective action is necessary to make the whole process work more efficiently.
Action on the Process
Action on the process is very important for any process, since it prevents the production
of an out-of-specification product. An action on the process could be a
change in raw material, operator training, equipment, design, or other measures.
Effects of such actions on a process should be monitored closely and further action
should be taken if necessary.
Action on Output
If the process evaluation does not indicate that any action on the process is necessary,
the output is to ship the final product to its destination. Note that some
people believe action on output consists of sampling plans and discarding out-ofspecification
product that has already been produced. Obviously, such an action
on the output is futile and expensive. We are interested in correcting the output
before it is produced. This goal can be achieved through the use of control charts.
Part IV.B.1
Variation
No process can produce two products that are exactly alike or posses exactly the
same characteristics. Any process is bound to contain some sources of variation. A
difference between two products may be very large, moderate, very small, or even
undetectable depending on the source of variation, but certainly there is always
some difference. For example, the moisture content in any two rolls of paper, opacity
in any two spools of paper, the brightness of two lots of pulp, will always vary.
Our aim is to trace back the sources of such variation and as far as possible to
eliminate them. The first step is to separate the common and special causes of the
sources of variation.