Page 332 - The Handbook for Quality Management a Complete Guide to Operational Excellence

318 C o n t i n u o u s I m p r o v e m e n t A n a l y z e S t a g e 319

A good cause-and-effect diagram will have many “bones.” If your
cause-and-effect diagram doesn’t have a lot of smaller branches, then the
understanding of the problem is somewhat superficial.
Cause-and-effect diagrams come in several basic types. The disper-
sion analysis type is created by repeatedly asking “Why does this disper-
sion occur?” For example, we might want to know why all of our fresh
peaches don’t have the same color.
The production process class cause-and-effect diagram uses production
processes as the main categories, or branches, of the diagram. The pro-
cesses are shown joined. Other common themes in cause-and-effect dia-
grams include those with main branches designated using the 5M and E
(machine, method, manpower, material, measurement, and environment)
and the 4P (people, policy, procedure, plant).
An example is shown in Fig. 15.3.
The cause enumeration cause-and-effect diagram simply displays all
possi ble causes of a given problem grouped according to rational catego-
ries. This type of cause-and-effect diagram lends itself readily to the brain-
storming approach.
Cause-and-effect diagrams have a number of uses. Creating the dia-
gram is an education in itself. Organizing the knowledge of the group
serves as a guide for discussion and frequently inspires more ideas. The
cause-and-effect dia gram, once created, acts as a record of your research.
Simply record your tests and results as you proceed. If the true cause is
found to be something that wasn’t on the original diagram, it should be
added. Finally, the cause-and-effect diagram is a display of your current
level of understanding. It is a good idea to post the cause-and-effect dia-
gram in a prominent location for handy reference in the future.
A variation of the basic cause-and-effect diagram, developed by
Dr. Ryuji Fukuda of Japan, is cause-and-effect diagrams with the addition
of cards, or CEDAC. The main difference is that the group gathers ideas
outside of the meeting room on small cards, as well as in group meetings.
The cards also serve as a vehicle for gathering input from people who are
not in the group; they can be distributed to anyone involved with the pro-
cess. Often the cards provide more information than the brief entries on a
standard cause-and -effect diagram. The cause-and-effect diagram is built
by actually placing the cards on the branches.

Scatter Diagrams
A scatter diagram is a plot of one variable versus another: the independent
variable is shown on the horizontal (bottom) axis; the dependent variable is
shown on the vertical (side) axis.
Scatter diagrams are used to evaluate the correlation of one variable
with the other. The premise is that the independent variable is causing
a change in the dependent variable, although strictly speaking cause and
effect cannot be proven with statistics alone. Scatter plots are used to

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