Also known as Ishikawa Diagram, Cause and Effect Diagram and Fishbone Analysis.
Variants include Tree Diagram.
A fishbone diagram is one of the various tools that can be used to perform a cause-and-effect analysis. It provides a structured way to identify and organize the potential causes of a problem (or effect). It allows to establish and present the cause-and-effect relationship in an easy and understandable format. The word fishbone comes from its visual representation that looks like the skeleton of a fish.
A fishbone diagram is one of the seven basic tools of quality. It helps to identify the root causes of an effect in order to identify an appropriate solution that can be effective. It is often used in Lean Six Sigma and other quality improvement methodologies to narrow down the area of analysis. The outcome of the fishbone analysis will provide useful information for later problem-solving tools.
A fishbone diagram can also be used in product and process design, and as a cause prevention tool to prevent future problems and risks.
The first step in conducting a fishbone analysis is to clearly define the effect. This effect could be a quality issue, a technical issue, or simply not meeting a performance target. The effect becomes the head of the fishbone while the causes become the branches. For easier sharing and reference, each cause needs to be put into a category. There are many ways that can be used to categorize the causes such as the 6 Ms which we will talk about later.
Combining the fishbone with the 5 whys will give the analysis an extra dimension. This allows to identify the hierarchy of causes including the possible root causes. Root causes are normally those at the ends of the chains of causes that do not have any sub-causes. The main idea behind the fishbone analysis is to brainstorm all possible causes that may contribute to an effect, so be aware of brainstorming and adding causes which are actually solutions.
The 6 Ms Approach
6 Ms is one of the common approaches that can be used to categorize the potential causes and label the different branches on the fishbone diagram. It is often used in manufacturing and production environments as well as in service environments. These categories typically include Man, Method, Machine, Material, Environment, and Measurement.
There are many other categorization approaches that might be worth considering. For example, the 8 Ms will add Management and Maintenance to the 6M categories. Another example is when the effect results from a documented or mapped process, the process steps can be used as the basis for categorization. In this case, the process input variables can be considered to be the potential causes.
Different names may be used for the same category depending on the situation and on the user’s choice.
Conducting the Analysis and drawing the Fishbone
A fishbone analysis is a teamwork exercise that allows to capture, categorize and organize people’s knowledge of a process. By going through the steps of drawing the diagram with your team, everyone gains insight into the cause-and-effect, which makes the solution easier to find.
- With your team, clearly define the effect that you want to work on.
- Write the effect statement in a box at the center right of a large piece of paper.
- Draw a long horizontal line pointing to the box.
- Identify the cause categories using the 6 Ms or any other approach.
- Write them in boxes parallel to the horizontal line.
- Use brainstorming to log all possible causes under the appropriate categories. Start with the main causes then the secondary causes and so on.
- Update the chart as new potential causes become apparent.
- Highlight or assign numbers near to the key causes to show their relative importance.
- If necessary, collect data to confirm key causes are real.
Note: A cause-and-effect matrix can be used to prioritize the causes of the problem. Prioritizing and selecting the key causes will minimizes the need for more statistical analysis of inputs that are unlikely to have an impact on the output.
Example – Manufacturing
The example shown here is the output of a brainstorming session on the causes of the increased coolant consumption in a manufacturing plant. Note that this fishbone diagram is labeled with the 6 Ms.
Example – Non-Manufacturing
The example shown here is the output of a brainstorming session on the causes of the increased invoice errors for a particular company. Note that only the main causes has been displayed.
Example – Fishbone Combined with 5 Whys
A restaurant manager has noticed an increase in the number of customer complaints regarding the taste of the coffee they make. His team has constructed a fishbone diagram and added the possible causes and their sub-causes for only two categories.
Another way of highlighting the structure of the possible causes is to use the why-why diagram.
There are many tools that can help you to identify and present your cause-and-effect. One of the simplest ways is to use this fishbone diagram template.
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