Fishbone Diagram

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Purpose

The Fishbone Diagram, also known as the Cause and Effect Diagram or Ishikawa Diagram, is a graphical tool used to identify and explore on a single chart, in increasing detail, the possible causes which lead to a given effect.

They are a structured framework that takes advantage of the collective knowledge of a team to identify the main causes of the effect under study.

The ultimate aim is to work down, or drill-down, through the causes to identify basic root-causes of a problem.

The structure provided by the diagram helps team members think in a very systematic way. Some of the benefits 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.
  • Identifies areas where data should be collected for further study.

Constructing a Fishbone Diagram (Cause and Effect Diagram) can help a team when it needs to:

  • Identify the possible root causes, the basic reasons, for a specific effect, problem, or condition.
  • Sort out and relate some of the interactions among the factors affecting a particular process or effect.
  • Analyze existing problems so that corrective action can be taken.

Anatomy

When you develop a Fishbone Diagram (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.

The picture below shows the basic layout of diagram. Notice that the diagram has a cause side and an effect side. The steps for constructing and analyzing a Cause and Effect Diagram are outlined below.

Fishbone Diagram

Fishbone Diagram showing major and sub-cause categories.


Reference: The Memory Jogger

Terminology

A. Major Cause Categories.

B. High level cause.

C. Root cause, i.e. cause of a cause.

D. Secondary root cause, i.e. cause of a root cause.

E. The effect of the causes, i.e. the problem whose causes is being investigated.


Major Considerations

The Major Cause Categories are not firmly defined, and can easily vary according to the situation, or the type of problem being studied.

For example, six categories are typically used in manufacturing processes:

  • Material
  • Machine
  • Measurement
  • Method
  • Manpower
  • Environment

Similarly, four are typically used with administrative processes:

  • Personnel,
  • Plant Facilities,
  • Policies and,
  • Procedures.

Depending on the situation, other categories are possible. Fishbone Diagrams are best prepared in a team setting using Brainstorming techniques, but can also be based on process data if it is available. The same cause should not be used on several exercises.

Application Cookbook

1. Select the Problem Statement, or Effect, summarized in a few key words, and place it in a box on the right side of the new diagram.

Notes:

  • 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 and sometimes enhances group participation.

2. Select the Major Cause Categories, according to the specific situation and problem statement, and connect them with a straight line to the “backbone” of the diagram.

3. Place brainstormed or data-driven causes in the appropriate Category.

4. Place Root Causes, against each of the main causes.

5. Continue driving down, identifying further lower-level Root Causes.

From Fishbone Diagram to Six Sigma Tools.

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