The fishbone diagram is a popular method that can be used to identify and analyze possible causes of a problem. The fishbone is a simple visual tool to structure in a graphical way the possible causes of a problem. It collects the different ideas of the team and put them together into categories. The fishbone diagram is often also called the Ishikawa diagram or cause and effect diagram.
The fishbone is created by drawing a straight horizontal line, which is the backbone of the fish. On the right side, the fish's head, a short description of the problem is stated. Then the different categories are drawn as ribs branching off from the backbone of the fish.
The final diagram looks like the bones of a fish, hence the name fishbone diagram.
Fishbone categories: 6M, 8P, 4S,...
Which categories you use depend on the problem you want to solve. In manufacturing often the 6M are used: Manpower, Machine, Material, Method, Measurement, Mother Nature:
- Manpower refers to the people who are executing or interacting with the process
- Machine refers to the tools, equipment, or machines that are used during the process
- Materials are the raw materials, semi-finished and finished products that are used by and transformed within the process
- Method refers to the way things are done within the process. It can be written or unwritten rules.
- Measurements refer to the way product and/or process parameters are measured during the process.
- Mother Nature is the immediate environment in which the process takes places
The 6M categories are the most frequently used when making a fish-bone diagram. But there are other types.
In product marketing the 8P are used: Product , Price, Place, Promotion, People, Process, Physical evidence, Performance
And is service industry you have the 4S: Surroundings, Suppliers, Systems and Skill
As you gain more experience with creating fish-bones you can remove and add categories to fit them more to your specific case. possible categories you could think of are management, mission, maintenance, ...
Each category identify a possible type of variation. The categories should covers all the fields of variation with your process.
Drafting a fishbone diagram
Typically the fishbone is made on a whiteboard or flip chart. You can use sticky notes to write down your ideas and place them on the fishbone. The benefits of using sticky notes are that you can move them around easily later on. Of course, you can write them down also directly on the diagram. Try to place the board physically as close as possible to where the problem occurs.
You should define a multi-disciplinary team in order to have all the viewpoints on the problem. It increases the chances of finding the real root cause of your problem. this way the fishbone has more value than if one person alone would create it. don't forget the people who are executing the work in the process. The fishbone diagram gives the team a framework to hold a brainstorming session in a structured way.
Before you start brainstorming the possible causes you have to make certain that everyone understands the problem. If you made a 6W analysis you can run over it again with the team so that everyone is on the same page. Draw the fishbone, explain the categories to the team if need and start brainstorming possible causes.
Filling up the categories
start with one category and let all the participants give their ideas. If there are no more ideas, move to the next category. By doing so the team can organize the possible causes in a logical way and it makes certain that all types of variation within your process have been covered. Note that some causes could be placed under different categories.
Do not spend too much time discussing in which category you should place the cause, the main idea is that you have the cause visualized somewhere on the board. The final diagram should have all the main causes and subcauses which could possibly cause the problem. Each cause is a potential source of variation. Once you have finished the fishbone diagram we can take some time to review the diagram and discuss what the most likely causes are. Once you agree on them, underline them clearly on the fishbone and start further analyzing them.
When to use a fishbone diagram
A fishbone diagram can be used during different phases of the DMAIC process. in the Define phase, you can use the fishbone on the general problem statement. during the measurement phase, you can use the created fishbone diagram to define what you are going to measure. And during the Analyze phase you can create new fishbone diagrams on specific issues.
Let's illustrate that with an example. Imagine you are running a pizza delivery service and have a problem that many pizzas are not delivered on time. During the measurement phase, you find out that late deliveries are done mainly by new deliverers. No, you can do a fishbone to find all the possible causes why the new deliverers are delivering too late.
When using a fishbone you can visualize all the possible causes at once, which makes it easy to present your finding to the different stakeholders. Be aware that when analyzing a complex problem the diagram could become visually cluttered.