How to Determine Root Cause Using a Fishbone Diagram


The fishbone diagram is a tool you can use to determine the root cause of a problem or event. The fishbone diagram is also known as the Ishikawa diagram or cause and effect diagram and gets its name because its shape resembles a fishbone. The fishbone diagram was developed as a quality management tool for categorizing and brainstorming the possible causes of a problem in an organized manner.

In order to have an effective brainstorming session to consider all of the variables and possible causes, you will need to assemble a team of people from different functional groups, with varied specialties and experience.

Having a diverse group of people means gaining insight from different viewpoints and opinions. Approaching the problem from different angles could lead to answers that you may not have thought of on your own, and ultimately reveal the root cause of the issue.

Draw the basic structure of the fishbone diagram.

Write down the problem or effect that you are trying to understand (the head). Draw a main line from this problem with several branches to represent categories (the bones). From each of these category branches, draw horizontal lines to represent causes. You can also find ready-made templates online for easy use.

Determine which categories are most appropriate for your situation to be used as the branches of the fishbone diagram. Some commonly used categories are:

The 6 M's (in the manufacturing industry) Machine, Method, Materials, Measurements, Man and Mother Nature

The 8 P's (in the services or administrative industries) Price, Promotion, People, Processes, Place, Policies, Procedures, and Product

The 4 S's (in the service industry) Surroundings, Suppliers, Systems, Skills

Once you have the categories labeled on the branches, start to fill in any possible causes on the horizontal lines coming from each branch. Fill in anything that could possibly have an influence on the issue. This is not the time to decide if an idea is good or not, just brainstorm to collect any possible causes.

Evaluate each of the causes on each branch of the fishbone diagram individually and in more depth to determine what variables affect it. You want to determine what the impact is of changing those variables and if the change could lead to the effect or problem you are investigating.

Do this for each cause, eliminating those that would not have any possible impact on the problem or lead to the identified effect.

Now you should be able to narrow the causes down to one or at most a few likely candidates. Focus your investigative efforts on those possibilities until the true root cause of the issue is determined.

Tips & Warnings

  • Use the 5 Why Process in conjunction with the Fishbone diagram to find more possible causes for the event.

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