iSixSigma

Fishbone Diagram

Definition of Fishbone Diagram:

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The three phases of the creative process are idea generation, organization, and assessment. Brainstorming is the primary tool for generating ideas. Affinity diagrams are used to do a high-level organization of the ideas and the fishbone diagram is used to drill down and do root cause analysis. This is typically done in the Analyze phase of the Six Sigma DMAIC process.

Overview: What is a fishbone diagram? 

Fishbone diagrams, also known as Cause and Effect Diagrams, Ishikawa diagrams and the 6 Ms, were popularized in the 1960s by Kaoru Ishikawa, who pioneered quality management processes in the Kawasaki shipyards. The basic concept was first used in the 1920s and is considered one of the seven basic quality control tools.

The graphic below shows the format for a fishbone diagram:

The head of the fish represents the problem being addressed. The main bones are the major categories of possible root causes. The smaller bones represent the cascading and drilling down of possible root causes.

The process for creating a fishbone diagram is as follows:

  • Name the problem or effect of interest.
  • Decide the major categories for causes. This can come from the themes in your affinity diagram.
  • Typically, your fishbone diagram will have 4-6 main bones representing the major causes of what you are analyzing.
  • Major causes may include the 6 M’s: manpower (or personnel), machines, materials, methods, measurements, and mother nature (or environment).
  • Brainstorm for more detailed causes.  Ask why each major cause happens at least 5 times.
  • Eliminate causes that do not apply.
  • Discuss the causes and decide which are most important. If you have existing data or can collect data on the potential causes, then you can make objective rather than subjective decisions about the validity of that potential root cause.
  • Work on the most important root causes.
  • Perform another iteration to determine root causes if necessary.

An industry example of a fishbone diagram 

A software company was concerned about losing business to their competitors. They put an improvement team together to try and understand what the causes of their reduced competitiveness might be. The Six Sigma Master Black Belt assigned to the team held a brainstorming session to try and identify potential root causes.

After gathering all the ideas, the team developed an Affinity Diagram and identified several themes or possible root cause categories. Since this was not a manufacturing problem, they selected more appropriate transactional categories. The graphic below shows the outcome of their root causes analysis and application of a fishbone diagram.

Fishbone diagram

Frequently Asked Questions (FAQ) about a fishbone diagram

1. Can I use the fishbone diagram for non-manufacturing problems? 

Yes. While you will frequently use the fishbone diagram in a manufacturing environment, the concept of root cause analysis can apply to non-manufacturing and transactional processes as well. Since Machinery and Material may not be as applicable to a non-manufacturing process, you might see the use of People, Policies, Processes, Training, and Communication as the primary bones of the fishbone diagram.

2. Are other names used for the fishbone diagram? 

Yes. It is also known as a Cause and Effect Diagram, 6M Diagram or an Ishikawa Diagram named after the person who developed the concept.

3. What is the main purpose of a fishbone diagram? 

A fishbone diagram is a graphical tool used to identify, sort, and display possible causes of a specific problem. It shows the hierarchical relationship between an outcome or effect and all the possible factors or causes which may influence the outcome.

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