Cause-and-Effect Diagrams

[Steven Bonacorsi]

It is difficult if not impossible to solve complicated problems
without considering many factors and the cause-and-effect
relationships between those factors. Defining and displaying those
relationships helps. The first such cause-and-effect diagram was used
by Kaoru Ishikawa in 1943 to explain to a group of engineers at the
Kawasaki Steel Works how various work factors could be sorted and
related. In recognition of this, these diagrams sometimes are called
Ishikawa diagrams. They are also called fishbone diagrams, because
they look something like fish skeletons.

What can it do for you?

Quality problems are typically not simple. They often involve the
complex interaction of several causes. A cause-and-effect diagram will
help you:


•Define and display the major causes, sub-causes and root causes that
influence a process or a characteristic.
•Provide a focus for discussion and consensus.
•Visualize the possible relationships between causes which may be
creating problems or defects.


Cause-and-effect diagrams are particularly useful in the measure and
improve phases of Lean Six Sigma methodology.

How do you do it?


1.Decide which quality characteristic, outcome or effect you want to
examine. You might consider Pareto analysis to help you focus on the
most important issue.
2.Write your chosen effect on the right side of a paper, board or
flipchart and draw a box around it. If you think of this as a fishbone
diagram, this is the fish head.
3.Draw a straight line to the left, the fish backbone.
4.For each primary cause or category of causes, draw a diagonal line
slanting from left to the centerline. Alternate these ribs on the top
and bottom of the backbone. Label the end of each rib and draw a box
around the label.
5.Draw a horizontal line intersecting the appropriate diagonal line
and label it to describe each secondary cause that influences a
primary cause. Alternate these medium sized bones to the left and
right of each rib.
6.In a similar way, draw and label diagonal lines for third level or
root causes, small bones, intersecting the secondary cause lines,
medium sized bones.
7.Examine the diagram. If certain causes seem to have a significant
effect on the characteristic you are examining, mark them in a special
way.


Variation 1: Cause Enumeration

Sometime it may be very difficult to determine the primary causes to
be included in your diagram. If that is the case, after you have
determined the characteristic or effect you are examining, follow
these steps:


•Use brainstorming to create a list of all the possible causes. The
list will contain a mixture of primary, secondary and tertiary (or big
bone, middle sized bone and small bone) causes.
•Sort the list by grouping causes that are related.
•Identify or name each major grouping and make your cause-and-effect
diagram.
•Machine, Manpower, Material, Measurement, Method and Environment are
frequently used major causes that can apply to many processes.


The advantage of the cause enumeration technique is that you stand a
much better chance that all causes will be listed, especially hidden
ones, and your diagram will be a complete and useful picture. The
disadvantage is that it may be difficult to relate all the causes
clearly to the result, making the diagram hard to draw.

Variation 2: Process Classification

Sometimes it is more helpful to look at causes in the sequence in
which they occur instead of considering overreaching logical
categories. With this approach, the center line or backbone follows
the sequence of the process.


•Instead of primary causes as the ribs, show the major process steps
from left to right.
•Construct your cause-and-effect diagram as before.


The advantage of this technique is that, since it follows the sequence
of the process, it will be easy for everyone to understand. The
disadvantages are that similar causes will appear again and again, and
causes due to a combination of factors will be difficult to show.

Hints for Making Good Diagrams


1.Get input from many people involved in the process. Not only will
this make for a more accurate diagram, everyone taking part will gain
new knowledge.
2.Make one cause-and-effect diagram for each Critical-To-Quality
(CTQ) characteristic you are considering. Trying to include all CTQs
on one diagram will make it too large and complicated to be of much
use as a problem solving tool.
3.Avoid generalities. Express each cause as concretely as possible.
4.Since you will use your diagram to direct the examination of
specific cause-and-effect relationships with data, the characteristic
you are considering and all the causal factors should be measurable.
If they are not, try to make them measurable or find substitutes.
5.The objective of the cause-and-effect diagram is action. Be sure
your causes are broken down to the level at which they can be acted
on.


Now what?

· Use your diagram to develop a common understanding of the factors
potentially influencing or causing a quality problem.

· Use your diagram as a road map for collecting data to verify the
causal relationship of various factors to the characteristic.

· Continue to annotate and modify your diagram as you verify
relationships and learn more.

Using a cause-and-effect diagram this way will help you to see which
factors in your process need to be checked, modified or eliminated.


Steven Bonacorsi is a Certified Lean Six Sigma Master Black Belt
instructor and coach. Steven Bonacorsi has trained hundreds of Master
Black Belts, Black Belts, Green Belts, and Project Sponsors and
Executive Leaders in Lean Six Sigma DMAIC and Design for Lean Six
Sigma process improvement methodologies.

Author for the Process Excellence Network (PEX Network / IQPC)

Process Excellence Network
Steven Bonacorsi, President of International Standard for Lean Six
Sigma(ISLSS)
Certified Lean Six Sigma Master Black Belt
47 Seasons Lane
Londonderry, NH 03053
Phone: +(1) (603) 401-7047
E-mail: sbona...@islss.com
Process Excellence Network: http://bit.ly/n4hBwu

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