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A fishbone diagram is a cause-and-effect discovery tool that helps figure out the reason(s) for defects, variations or failures within a process. In other words, it helps break down, in successive layers, root causes that potentially contribute to an effect. Sometimes called an Ishikawa diagram or cause-and-effect analysis, a fishbone diagram is one of the main tools used in a root cause analysis.
A fishbone diagram, as the name suggests, mimics a fish skeleton. The underlying problem is placed as the fish’s head (facing right) and the causes extend to the left as the bones of the skeleton; the ribs branch off the back and denote major causes, while sub-branches branch off of the causes and denote root causes. These causes mimic the bones of the fish skeleton. The construction of the fishbone can branch off to as many levels as is needed to determine the causes of the underlying problem.
The concept of the fishbone diagram is said to have been around since the 1920s, but it was popularized by Japanese professor of engineering, Kaoru Ishikawa, who, most notably, put into place the quality management processes for the Kawasaki shipyards. One of the more famous uses of the fishbone diagram wasn’t used to find causes of existing problems, but rather in the design phase to prevent problems. Mazda Motors used fishbones diagrams to design the Miata (MX5) sports car. Details down to the design of the car’s doors, so drivers could rest their arm on it while driving, were considered.
In nearly any industry or business, variation is quality’s and efficiency’s kryptonite. Once an order has been placed, a predictable and standardized process should be followed through to delivery. Variation in that process usually means variation in the final product.
Fishbone diagrams are a great tool to either determine potential variables in a process to mitigate defects or failures, or to figure out what’s causing a defect or failure that is currently occurring.
As we dive into when and how to use a fishbone diagram, it’s important to note the advantages and disadvantages that can present themselves when using this root cause analysis tool. Advantages go in accordance with the purpose and method of the diagram.
Some disadvantages that can present themselves relate to the greatest strengths of the fishbone diagram.
|Advantages and Disadvantages of Fishbone Diagrams|
|Helps identify cause and effect relationships||Irrelevant potential causes can cause confusion|
|Helps develop in-depth joint brainstorming discussion||Complex issues may lead to a messy diagram|
|Works well with the 5 Whys to drill down to a root cause quickly||Lends itself to the divergent approach|
|Helps prioritize relevant causes, allowing you to address underlying root causes||Based on opinion rather than evidence; it needs testing to prove results|
One of the first steps in creating a fishbone diagram is determining the factors that contribute to variations within a process. Ishikawa describes these contributing factors as the 6 Ms in the manufacturing world: man, machine, method, material, measurement and Mother Nature. These 6 Ms influence variation in all processes and serve as the first six main “bones” of your fishbone.
Let’s take a look at how the 6 Ms are defined and how they can contribute to process variation.
The 6Ms are often used as a starting point for the spine of a fishbone diagram (which denotes the potential causes of the problem statement). There won’t always be six causes, so you can add or subtract them as necessary. Once you have the basic structure of the fishbone diagram, you and your team can collaborate and identify the various causes affecting the final outcome. This is very often done using the 5 Whys technique.
You’ll often hear of the fishbone diagram being used closely with another common root cause analysis tool: the 5 Whys. Both of these tools, although different in how they’re executed, help get to the root cause of a problem quickly and figure out why the problem is happening. They also complement each other nicely when determining the cause and effect relationship of an issue because they determine the cause (why something happens) and the effect (what happens).
The 5 Whys method, like the fishbone diagram, starts with an underlying problem statement and then proceeds to ask the question “why?” five times. Each answer to a “why?” question forms the basis of the next question. For example, if your problem statement reads, “my car stopped in the middle of the road,” you’d ask, “why did my car stop?” Because it ran out of gas; “why did it run out of gas?” Because I didn’t buy any on my way home; “why didn’t I buy any gas on my way home?” Because I didn’t have any money; “why didn’t I have any money?” Because I lost it all gambling; and so on until you get to the root cause.
So how do the two relate? The 5 Whys can be used as an individual tool or in tandem with a fishbone diagram. When using it with a fishbone diagram, once you’ve established the main causes (generally the 6 Ms), you can use the 5 Whys to drill down on various root causes that form the sub-branches off of each branch. Asking “why?” is essentially what your brainstorming session involves when creating a fishbone diagram. When you ask why a cause happened, you write it as a branch stemming from the appropriate category.
For example, say your fishbone diagram has a sub-cause of “lighting” under the “Mother Nature (environment)” category of the 6 Ms; however, it’s not clear how lighting can lead to a failure. In this instance, lighting should be investigated further – what if the lighting caused a technician to make a mistake resulting in a part not being installed properly? If that’s the case, the part not being installed properly would be listed on the fishbone diagram under the “method” category of the 6 Ms.
In other words, if a part isn’t correctly installed, use the 5 Whys on that part of the fishbone diagram to investigate further. In this case, the lighting may be a contributing factor, but it shouldn’t take priority over a part not being installed correctly. Each time the 5 Whys is used, expand the fishbone diagram. For instance, you may end up with a branch of the fishbone diagram looking like this: Material – part not installed correctly – employee skipped a step – work area too dark – poor lighting – light bulb is burned out.
In this example, the 5 Whys helped fill out a section of the fishbone diagram properly by leading to the true cause of the failure: the light bulb being burned out. In this case, you can retrain employees all you want but the same employee or anyone else, for that matter, may make the same mistake or another mistake if the poor lighting issue isn’t corrected.
The fishbone diagram technique combines brainstorming and mind mapping to discover the cause and effect relationship of an underlying problem. It pushes you to consider nearly every possible cause of an issue instead of just getting stuck on the most obvious ones. Not only does it help you get to the root cause of a problem, it also uncovers bottlenecks in your processes and identify areas where your processes aren’t working. Let’s take a look at how to create a fishbone diagram is four steps using an example.
1. Identify the problem.
The first step in creating a fishbone diagram is agreeing upon and writing down the exact problem you’re facing. Have your team figure out who’s involved, what the problem is and when and where it occurs.
Next, on a whiteboard, write the problem statement in a box on the right-hand side and draw a horizontal line protruding left from the problem statement. The problem statement resembles the head of the fish and the horizontal line resembles the spine, giving you room to develop ideas.
Our example below identifies a problem of a maintenance technician slipping and injuring himself during a routine inspection.
2. Brainstorm the major categories of causes.
Step two is deciding how to categorize the causes of the problem at hand – the factors that may be part of the problem. These could be equipment, materials, systems, external forces, people, etc. In a manufacturing setting, they usually revolve around the 6 Ms we discussed earlier.
As you identify each cause, draw a line off the spine of the fish for each factor and label each line at the top. In our example, the team identifies the following factors and places them on the diagram:
3. Identify possible causes.
Step three really kicks off the brainstorming session as you identify possible causes of the problem that may be related to each factor. Visualize these possible causes by drawing shorter lines off of the “bones” of the diagram. If a particular cause is a bit more complex, feel free to draw smaller sub-branch lines coming off of the “cause” line.
Below, the team identifies the following causes and adds them to the fishbone diagram, slowly filling out the skeleton of the fish.
4. Analyze the diagram.
By now, you should have a full fishbone diagram showing all possible causes of the problem statement. Using tools like the 5 Whys, investigations and surveys, you can now investigate the problem further and test to see which of these potential causes is, in fact, contributing to the problem.
In our example, the team has determined the most likely cause of the problem is the service manuals need to be updated to specify which type of seal and seal material needs to be ordered for the machine in question. On the surface, it looked as if the cause might have been related to improper maintenance practices that lead to a leak.
A helpful tip for this step is to write all possible cause of the problem on sticky notes, so you can group similar ones together on the whiteboard without having to erase and rewrite so much.