Cause-and-Effect Diagram
A cause-and-effect diagram is an analysis tool that provides a systematic
way of looking at effects and the causes that create or contribute to
those effects. It was develop by Dr.
Kaoru Ishikawa of Japan in 1943 and is sometimes referred to as
an Ishikawa Diagram or a Fishbone Diagram because of its shape.
When should we use a cause-and-effect diagram?
A cause-and-effect diagram is a tool that is useful for identifying
and organizing the known or possible causes of quality, or the lack
of it. The structure provided by the diagram helps team members think
in a very systematic way. Some of the benefits of constructing a cause-and-effect
diagram are that it:
- helps determine the root causes of a problem or quality characteristic
using a structured approach;
- encourages group participation and utilizes group knowledge of
the process;
- uses an orderly, easy-to-read format to diagram cause-and-effect
relationships;
- indicates possible causes of variation in a process;
- increases knowledge of the process by helping everyone to learn
more about the factors at work and how they relate; and
- identifies areas where data should be collected for further study.
How do we develop a cause-and-effect diagram?
When you develop a cause-and-effect diagram, you are constructing a
structured, pictorial display of a list of causes organized to show
their relationship to a specific effect (Viewgraph
1). Notice that the diagram has a cause side and an effect side.
Steps in constructing and analyzing a cause-and-effect diagram:
| STEP 1: |
Identify and clearly define the outcome or
EFFECT to be analyzed. |
- Decide on the effect to be examined. Effects are stated as particular
quality characteristics, problems resulting from work, planning objectives,
and the like.
- Use operational definitions. Develop an operational definition
of the effect to ensure that it is clearly understood.
- Remember, an effect may be positive (an objective) or negative
(a problem), depending upon the issue that is being discussed.
- Using a positive effect which focuses on a desired outcome tends
to foster pride and ownership over productive areas. This may lead
to an upbeat atmosphere that encourages the participation of the
group. When possible, it is preferable to phrase the effect in positive
terms.
- Focusing on a negative effect can sidetrack the team into justifying
why the problem occurred and placing blame. However, it is sometimes
easier for a team to focus on what causes a problem than what causes
an excellent outcome. While you should be cautious about the fallout
that can result from focusing on a negative effect, getting a team
to concentrate on things that can go wrong may foster a more relaxed
atmosphere which sometimes enhances group participation.
You must decide which approach will work best with your group.
| STEP 2: |
Use a chart pack positioned where everyone
can see it. Draw the SPINE and create the EFFECT box. |
| STEP 3: |
Identify the main CAUSES contributing to the
effect being studied. |
These are the labels for the major branches of your diagram and become
categories under which to list the many causes related to those categories.
- Establish the main causes, or categories, under which other possible
causes will be listed. You should use category labels that make sense
for the diagram you are creating. Here are some commonly used categories:
- 3Ms and P – Methods, Materials,
Machinery, and People
- 4Ps – Policies, Procedures,
People, and Plant
- Environment - a potentially significant fifth category
- Write the main categories your team has selected to the left of
the effect box. Draw some above AND below the spine.
- Draw a box around each category label and use a diagonal line to
form a branch connecting the box to the spine.
Example:
Viewgraph 3 uses the 3Ms and P to start developing the diagram
we began in Step 2.
| STEP 4: |
For each major branch, identify other specific
factors which may be the CAUSES of the EFFECT. |
| STEP 5: |
Identify increasingly more detailed levels
of causes and continue organizing them under related causes or categories.
You can do this by asking a series of WHY questions. |
Viewgraph 5 shows how the diagram looks when all the contributing
causes that were identified by the series of why questions have been
filled in. As you can see, there may be many levels of causes contributing
to the effect.
NOTE: You may need to break your diagram
into smaller diagrams if one branch has too many sub-branches. Any main
cause (3Ms and P, 4Ps, or a category you have named) can be reworded
into an effect.
| STEP
6: |
Analyze the diagram. |
Analysis helps you identify causes that warrant further investigation.
Since cause-and-effect diagrams identify only possible causes, you may
want to use a pareto chart to help your team determine the cause to
focus on first.
- Look at the “balance” of your diagram, checking for
comparable levels of detail for most of the categories.
- A thick cluster of items in one area may indicate a need for
further study.
- A main category having only a few specific causes may indicate
a need for further identification of causes.
- If several major branches have only a few sub-branches, you may
need to combine them under a single category.
- Look for causes that appear repeatedly. These may represent root
causes.
- Look for what you can measure in each cause so you can quantify
the effects of any changes you make.
- Most importantly, identify and circle the causes that you can take
action on.
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