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Fishbone Diagram

Fishbone Diagram

A fishbone diagram provides a structured way to identify and organize the potential causes of a problem. It allows to establish and present the cause-and-effect in an easy and understandable format. It is called this way because of its shape that looks like a skeleton of a fish. It is also called as cause-and-effect diagram and Ishikawa diagram after the man who created it. It is one of the seven basic tools of quality.

A fishbone diagram is used to identify the sources of variation within a process. It helps to identify the root causes of a problem or effect in order to identify appropriate solutions. It is often used in Lean Six Sigma and other quality improvement approaches to narrow down the area of analysis. The outcome of a fishbone diagram will provide further information for later problem-solving tools.

A fishbone diagram 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. 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 the fishbone analysis is to clearly define the effect which may be desirable or undesirable. This could be a quality issue, a technical issue, or not meeting a performance target. The effect becomes the head of the fishbone diagram while the causes will be placed in the branches of the diagram. Each cause needs to be put into a category for easier sharing and reference.

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 chains of causes that do not have any sub-causes. The main idea behind the fishbone diagram 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 service environments. These categories typically include Man, Method, Machine, Material, Environment, and Measurement.

The 6 Ms are:

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.

A third example is when you are working on an affinity diagram, you can use the titles of the affinity groups as the basis for categorization. Here, the items within each affinity group can be the potential causes. You may also rename or create your own branches that best suit your project, and there is no right or wrong way to do that.


Typical fishbone diagrams for manufacturing and non-manufacturing processes

How to Conduct a Fishbone Analysis

  • 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.

Example

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

The example shown here is the output of a brainstorming session on causes of the increased coolant consumption in a manufacturing plant. Note that this fishbone diagram is labeled with the 6 Ms.

Example

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.

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.

Further Information

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.


Another way of highlighting the structure of the possible causes is to use the tree diagram.


Different names may be used for the same category depending on the situation and on the user’s choice.


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