What is a Fishbone Diagram?

The Fishbone diagram leverages the logic of syllogism. Assuming you might not have your philosophy book handy, the following expression by Aristotle, maybe one of the most famous syllogistic statements of all time, might help illustrate:

“All men are mortal.
Socrates is a man.
Therefore, Socrates is mortal.”

The leaders of Athens went on to prove the point by forcing Socrates to drink a hemlock cocktail!

This is the logic of categories. The idea is that things can be grouped together by identifying shared characteristics or attributes. Kaoru Ishikawa, a twentieth-century Japanese quality expert, recognized that the industrial problems he was involved with would typically involve causes from these categories. A fishbone diagram is helpful in a root cause analysis because it organizes information in a consistent way that makes sense.

It gets its name from the fact that the diagram resembles a fish, with the problem at the head and various branches off of the trunk (bones).

How Does a Fishbone Diagram Work?

The logic works by drawing conclusions about different things or events based on shared traits and characteristics. Applied to answering the questions of why and how events occur, a Fishbone analysis defines standard high-level categories. The classic four (since expanded in a variety of ways, depending on application) are:
  • Manpower
  • Methods
  • Materials
  • Machinery

The classic Fishbone method works by brainstorming causes associated with each category, which results in sets of causes all sharing at least one similar trait which permits membership in the group. Are these categories set in stone? No – they can be changed and/or additional categories can be included. But the underlying logic – that things with at least one shared trait can be grouped together – is always consistent.

You may notice however that the Fishbone diagram, unlike the 5-Whys process, does not account for time. Though the presence of time is implied – it’s not explicit in the diagram. Therefore, in order to arrive at a “root cause,” the Fishbone process calls for investigation participants to vote on what they think the root cause is. Once determined, solutions can be adopted.

What are the Benefits of a Fishbone Diagram?

The primary benefit to the Fishbone is it helps the investigator identify causes from diverse sources. As long as there is access to a diverse team of knowledgeable experts, the Fishbone process will identify and group interesting potential causes together by category.

What are the Drawbacks to a Fishbone Diagram?

Fishbone does not take time into account. Most people find it helpful to see how an event unfolds over time. But the Fishbone process has no orientation to time – it only clusters like-causes together. It also does not show how different causes act together to result in an effect. Because the causes are separated into categories, the Fishbone diagram cannot show causal functions.

Does Sologic Use Fishbone?

The Sologic method uses syllogistic/categorical logic primarily during data gathering to help make sure that the investigation team has covered all their bases with respect to available evidence. But we do not use the Fishbone diagram itself. Causelink has the ability to assign any cause to a category, or “tag.” This is very helpful in terms of identifying a class of cause types that might be contributing to an excess of adverse events. Tags are also helpful in reporting. Causelink also will have the ability to generate Fishbone diagrams for those who prefer it.

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