Identify root cause (1) – Fishbone Analysis

Hello everyone. Last time, I wrote about “Value Stream Mapping (VSM)”, a method used in the “Measure” phase, which we can say is the “internal research” stage in Lean Six Sigma.

From this time, we will go into the “Analyze” phase, which we can say is the “internal research” stage in Lean Six Sigma. In Lean Six Sigma, the issues are identified on VSM in the “Measure” phase by clarifying detailed work flow and measuring the process time. And then, detailed issues are identified including the measured time. In the “Analyze” phase, the “root causes” of those detailed issues are investigated. I think this is the unique point of Lean Six Sigma when compared to the general flow of problem solving.

There are mainly 2 methods used in this “Analyze” phase, one is “Fishbone Analysis” and the other is “5 Whys”. So, this time I would like to write about “Fishbone Analysis”.

1. What is “Fishbone Analysis”? – It is also said as “Ishikawa Diagram”

“Fishbone analysis” is also called as “Ishikawa diagram” because it was developed by Dr. Kaoru Ishikawa, a Japanese professor of engineering. In Lean Six Sigma, it is generally called as “Fishbone analysis”. Because of this background, this might not be so familiar with problem solvers outside of the Lean Six Sigma community.

Seeing is believing, let’s see what it looks like.

Fig1. Fishbone analysis (Image) Source: Study Hacker

As you can see, the “fishbone analysis” is used to break down issues in a manner similar to a fish bone. On the head of the fish which is on the right side, write the “issue you want to explore”. Then, name each fish bone which is on the left side. Each bone is a “factor” of the issue.

You can find a lot of specific instructions on how to make it by googling, so let me just put one link here.

For those with good intuition may have guessed, it is important to set up these factors in “MECE” in order to conduct a good analysis. That’s where the “framework” comes in!

2. Frameworks to be applied to “Fishbone” – “4M”, “6M” and many others

The framework often used in fishbone analysis is the “4M” framework. Since the first letter of all four elements in English is “M”, so this is called as “4M”.





Actually, the image of a fishbone analysis with 4M applied looks like this (the red circles are the “root causes”).

Fig2. Fishbone analysis with “4M” applied (Image) Source: ROBOT Befriend

The following 2 may be added to 4M to make “6M”.


Mother Nature

In addition to “4M” and “6M”, “3C”, “4P” and other frameworks can also be used in various ways. Use your creativity according to the purpose of your analysis! In short, anything goes as long as it is “MECE”.

3. “Fishbone analysis” and “Logic tree”

The flow of breaking down the issue into “root causes” by factor in MECE is fine in itself, but personally, I don’t feel comfortable with the appearance of the fishbone. “Why is the head on the right?”, etc. I think one of the reasons is that if the head is on the right, which makes the whole analysis flow from “right to left”. Another is that our eyes wander all over the place because the factors are up and down. Generally speaking, it is natural for the human eye to move from the upper left to the lower right.

The solution to this is the “logic tree,” which is probably more familiar to problem solvers outside of the Lean Six Sigma community. I also personally feel more comfortable with this approach.

The same information as in Fig 2 above is shown here in Logic tree (the red lined boxes are the “root causes”).

Same contents with Fig2.  Logic tree version
Fig3. Same contents with Fig2. Logic tree version

Which is easier to see? Well, I think there are some preferences in this area. I think it is better to use whichever is easier to use or more comfortable for you!

That’s all for this time, and I would like to continue from the next time onwards. Thank you for reading until the end.

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