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Chapter 19: B. Statistical Process Control 249
The summary data not only gives the total number of different types of defects
but also provides a very meaningful source of trends and patterns of defects.
These trends and patterns can help find possible causes of any particular defect
or defects. Note that the column totals in Table 19.1 show the number of defects
(rolls of paper rejected) occurring daily, whereas the row totals will provide the
number of defects by type, occurring over the total period (four weeks) of study.
It is important to remark here that this type of data becomes more meaningful if
a logbook of all changes, such as changes of raw material, calibration of machines,
training of workers or new workers hired, and so on, is well maintained.
CAUSE-AND-EFFECT (FISHBONE OR ISHIKAWA) DIAGRAM
In implementing SPC, identifying and isolating the causes of any particular
problem(s) is very important. A very effective tool for identifying such causes is
the cause-and-effect diagram. Because of its shape this diagram is also known as a
fishbone diagram. Sometimes it is also known as the Ishikawa diagram after its inventor
Kaoru Ishikawa. Japanese manufacturers have used this diagram very widely
to improve the quality of their products. To prepare a cause-and-effect diagram
it is quite common to use a technique commonly known as brainstorming. Brainstorming
is a form of creative and collaborative thinking. Thus, this technique is
used in a team setting. The team usually includes personnel from departments
of production, inspection, purchasing, design, and management, and any other
members associated with the product under discussion. Some of the rules that are
used in a brainstorming session are the following:
1. Each team member makes a list of ideas.
2. The team members sit around a table and take turns reading one
idea at a time.
3. As the ideas are read, a facilitator displays them on a board so that all
team members can see them.
4. The method in step 2 and 3 continues until all of the ideas have been
exhausted and displayed.
5. Cross-questioning is allowed only for clarification concerning a team
member’s idea.
6. When all ideas have been read and displayed, the facilitator asks each
team member if he/she has any new ideas. This procedure continues
until no team member can think of any new ideas.
Once all the ideas are presented using the brainstorming technique, the next step
is to analyze them. The cause-and-effect diagram is a graphical technique for
analyzing these ideas. A typical initial structure of a cause-and-effect diagram is
shown in Figure 19.2.
The five spines in Figure 19.2 indicate the five major factors or categories that
could be the cause, or causes, of defect(s). In most workplaces, whether they are
manufacturing or nonmanufacturing, the causes of all problems usually fall into
one or more of these categories.
Part IV.B