Resources - Quality Systems

Fishbone Diagrams or Ishikawa Diagrams

Description The cause and effect diagram graphically represents the relationships between a problem (effect) and its possible causes. The development process is started in a group session led by a trained facilitator. The problem is stated in terms acceptable to the group. Possible causes are listed. The group then assigns priorities to the causes and action plans are developed.

When a cause and effect diagram is constructed, thinking is stimulated, thoughts are organized, and discussions are begun. These discussions bring out many possible viewpoints on the subject. Once all participants reach a similar level of understanding about an issue, an expansion of ideas can then be examined.

Cause and effect diagrams are developed in a form, commonly referred to as "fish," where the effect is found in a box to the right which is the head of the fish. The bones of the fish show the organized causes. The effects and causes can be expressed in words or data.

Application Cause and effect diagrams are used to examine many different topics which include the following:

  1. The relationships between a known problem and the factors that might affect it.
  2. A desired future outcome and its related factors.
  3. Any event past, present, or future and its causal factors.

The cause and effect diagram is useful in examining processes such as SPC, and SPC problems. The technique is also useful in planning activities and brainstorming. The diagram is basically a controlled way of gathering and using suggestions through group consensus.

Procedures A cause and effect diagram is developed in the following manner:

  1. Define the effect as clearly as is possible and place it at the head of the fish. This effect represents the "problem" that is being investigated. As data are collected, the effect can be redefined, if necessary.
  2. The group brainstorms the causes and lists them in no particular order. These causes are then studied and the causes that affect these causes are identified. This will continue until no new causes are thought of by the group.
  3. Once all causes are identified, list all categories, then display the categories on the diagram.
  4. The group then prioritizes the causes by multivoting. Each member of the group lists the causes in order of significance. Votes are counted and a final list is written.
  5. The highest prioritized causes are listed on the diagram as the big bones. The next highest prioritized causes will be listed on the diagram as the medium bones. Finally, the least prioritized causes will be listed on the diagram as the small bones.
  6. As categories and causes are included on the diagram, thinking may be stimulated and new causes may be identified.
  7. Teams are then formed to research and report on preventive (i.e., proactive) measures.

Advantages

  1. The cause and effect diagram enables quality analysis groups to thoroughly examine all possible causes or categories.
  2. The cause and effect diagram is useful in analyzing SPC problems. SPC detects a problem but can pose no solution.

Limitations The development of the cause and effect diagram can be time-consuming in order to arrive at a group consensus.

Source: System Engineering "Toolbox" for Design-Oriented Engineers, Sec 7. - NASA/RP-1358

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