Ishikawa diagram

The Ishikawa diagram is a management tool used in quality control as well as product development and manufacturing. It is also known as a cause-and-effect diagram, the herringbone diagram, or the fishbone diagram. The latter two names refer to the diagram’s appearance when it is drawn out. The diagram is also sometimes informally referred to by the nickname “Fishikawa diagram.” It is one of the seven basic quality control tools for process improvement and came into widespread use in the 1960s.

The diagram gives companies a way to find and correct problems in existing products and procedures and identify potential problems in new products. The multi-pronged appearance of the diagram allows members of a team to identify possible problems and their root causes. It provides a visual image of the potential issues so that many different areas of a company can understand the situation. The Ishikawa diagram also makes it easier to visually record potential causes that have been reviewed and eliminated, making it a way to track progress in the quality control process.

Background

The Ishikawa diagram was invented by Japanese statistician, college professor, and organizational theorist Kaoru Ishikawa. During the 1940s through the 1960s, Ishikawa was involved in developing a number of ways for Japanese companies to improve and maintain quality standards. He played a key role in development of the Total Quality Control (TQC) methodology and related aspects.

Ishikawa created the diagram that bears his name in the early 1960s. It was developed as a way of documenting and improving quality control for the Japanese shipbuilding industry. Its ease of use and versatility made it an invaluable tool for companies trying to identify the cause of problems, correct them, and prevent future problems. The diagram is one of the seven basic quality tools. The others are check sheets, control charts, histograms, Pareto charts, scatter diagrams, and stratification.

Overview

The Ishikawa diagram is most often used by a team of company employees to guide and document a brainstorming session to determine the cause of a problem or quality failure. The diagram is a visual way to capture the varied suggestions and information offered during brainstorming. It also provides a way to organize this input into categories, making it easier for team members to spot areas of weakness in the process. In addition, the visual representation of the information makes it possible for areas to be marked off as completed once they have been discarded as possibilities or resolved. As a result, it can be used throughout the entire review process.

Teams using the Ishikawa diagram will often use a paper flip chart, white board, or other method of creating a display the entire team can view. The team will first decide on a topic to investigate and list it on the display in as specific a format as possible. For example, if a car manufacturer has had repeated reports that a trunk latch is failing, the display might list something like “trunk latch on 2016–2018 model XYZ cars fails within nine months in sixty percent of models.” Being as specific as possible without theorizing on the cause is important to the accuracy of the results, so language such as “trunk latch is faulty” should be avoided. This allows for the possibility that it is not the latch that is faulty but something else causing the failure, such as the design of the trunk, variations in temperature, etc.

Once the problem has been agreed upon and listed, it is written to the right of the display and a line is drawn extending from the problem to the left side of the display. The team then begins to brainstorm possible causes. As team members offer suggestions, the leader lists them on diagonal lines as branches off of the main statement. These suggestions will generally fall into broad categories, such as the product materials, design, the machinery used to make them, human error, and the environment. In doing this, related aspects of the problem will begin to come together. For example, the “design” branch may include the design of both the latch and the trunk, while the “human error” category may indicate the possibility of errors in design and assembly.

The leader will then continue to ask questions that probe why each potential cause happens, creating sub-branches off the main branch. For example, in the “environment” category, team members might note that heat generated by the machine that shapes the latch could affect the material in a way that weakens it when it is exposed to cold temperatures. The leader continues asking questions until suggestions are exhausted. The process then focuses on asking questions in areas where there was little input to ensure nothing has been overlooked.

The resulting diagram will have a central idea at the head with branches radiating off as lines that resemble the ribs on the skeleton of a fish, thus giving the diagram its name. Once these ribs have been filled in, the company leadership will have a diagram that indicates the potential causes of the problem. The cluster of sub-branches will help identify any relationship between possible causes. For example, in looking at a cluster of potential causes, management might notice that all of the parts that fail are made using materials coming from one location while the parts that do not fail are using materials from another. This can help identify the root cause and suggest possible solutions.

While it is most often used to identify the source of problems, the Ishikawa diagram can also be used proactively. A company preparing to manufacture a new product can look at all the possible ways the product could fail as a way of preventing the problems. The diagram can also be used as a way to develop products or services that solve specific problems. For example, if a company wanted to find a new way to prevent the spread of disease borne by mosquitos, they might use the Ishikawa diagram to capture a brainstorming session on various ways to control the mosquito population as well as ways to keep them away from people.

Bibliography

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Barsalou, Matthew. “Root Cause Analysis, Ishikawa Diagrams and the 5 Whys.” ISixSigma, 2019, www.isixsigma.com/tools-templates/cause-effect/root-cause-analysis-ishikawa-diagrams-and-the-5-whys/. Accessed 10 June 2019.

“Fishbone (Ishikawa) Diagram.” American Society for Quality, 2019, asq.org/quality-resources/fishbone. Accessed 10 June 2019.

“How to Use the Fishbone Tool for Root Cause Analysis.” Centers for Medicare and Medicaid Services, www.cms.gov/medicare/provider-enrollment-and-certification/qapi/downloads/fishbonerevised.pdf. Accessed 10 June 2019.

“Kaoru Ishikawa: Developing a Specifically Japanese Quality Strategy.” American Society for Quality, 2019, asq.org/about-asq/honorary-members/ishikawa. Accessed 10 June 2019.

“Kaoru Ishikawa: One Step Further.” SkyMark, 2019, www.skymark.com/resources/leaders/ishikawa.asp. Accessed 10 June 2019.

Kenton, Will. “Ishikawa Diagram.” Investopedia, 26 Apr. 2018, www.investopedia.com/terms/i/ishikawa-diagram.asp. Accessed 10 June 2019.

Usmani, Fahad. “Fishbone (Cause and Effect or Ishikawa) Diagram.” PM Study Circle, 2019, pmstudycircle.com/2014/07/fishbone-cause-and-effect-or-ishikawa-diagram/. Accessed 10 June 2019.