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Cause and Effect Diagrams

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If you were asked to solve a problem, how would you go about it? Would you just start implementing a solution to solve the problem? How would you know if that solution is going to solve the problem? Would the problem be solved permanently, will the problem come back? The Cause & Effect diagram is a structured approach to graphically identify the cause of a problem before you devise means to solve the problem. In this module, we will look at why use the C&E diagram, who should be involved in creating it, when should you use this tool, and how to go about creating the C&E diagram.

Why should you use this tool?

Before you can solve a problem, you would first have to identify what is causing the problem. One of the tools you can use to identify the cause of the problem is called the Cause & Effect (C&E) diagram also known as the fishbone diagram as the diagram resembles the bones of a fish or the Ishikawa diagram after the person who came up with this method. C&E diagram is a visual way of representing the various causes that may be contributing to the problem. You can use this tool within a team environment to focus the team and capture the knowledge of the team. It can help the team visualize the different potential causes of the problem. The team can build on the diagram and ensure that the causes identified are comprehensive and the team is not missing any causes that should be considered.

Who should be involved?

The C&E diagram should be facilitated by a person who knows how to use this tool but it requires input from all team members because the knowledge of what is causing the problem best rests with the people involved with the process. Identify all the people who are aware of the process to get everyone’s point of view and the team’s collective knowledge to solve the problem.

When should you use C&E diagram?

The C&E diagram is a good tool to use to capture the subjective knowledge of the team members. Hence, it is a good starting point to attack a problem. This is one of the first tools that is used in the Analyze phase of a Six Sigma project. Use the C&E diagram anytime you are asked to solve a problem for which the causes are not known.

How do we create the C&E diagram?

Use the following steps to create the C&E diagram:
  • First write down the problem statement at the head of the fish. This should be a short one line description of the problem so that the team can focus on the problem in order to identify the causes.
  • Next, identify the main branches of the fish. There are a few different ways to determine the main branches. One option is to use the branches that best fit the process that you are trying to work on. You can first identify the causes and then use an affinity diagram to group similar ideas together and then find a suitable name for each group. A second option is to use the standard branches:
    • For a manufacturing process, you could use the branches: Man, Method, Machine, Material, Measurement, Milieu or Environment (also called the 6 M’s).
    • For a marketing process, you could use the branches: Price, Promotion, Product, Place (Distribution), People, Process (also called the 6 P’s).
    • For a service process, you could use the standard branches: Suppliers, Systems, Service, Skill, Surroundings (also called the 5 S’s).
    • For a business process, you could use the branches: Finance, HR, IT, Innovation, Purchasing, Production, Distribution, Sales & Service, Marketing, Government Relations.
  • For each branch identify why does that cause the problem. Keep asking why until you can find a cause that is actionable. It is important to identify a cause and not stop at a symptom (which is the result of a cause). Brainstorm with your team and identify as many causes as you can.
  • In order to emphasize the most important cause, discuss with your team and highlight the most important cause by drawing a cloud around it. You can also identify controllable and non-controllable causes by showing them in different colors. Controllable causes are causes that can be fixed by your team. Non-controllable causes are causes that are not in the control of your team or these causes are too difficult or expensive to control.
An example of the C&E diagram using the Sigma Magic software is shown below. Cause and Effect Diagram

Limitations of C&E Diagram

There are several limitations to the C&E diagram. Some of the key limitations are listed below:
  • C&E diagram is based on the subjective knowledge of the team. Hence, it is possible that not all causes may be identified and listed on this diagram by the team members.
  • Due to the subjective nature of this analysis, it is possible that the causes identified as the most important ones may in fact not be important or a cause at all in the real world.
  • If the process is non-stationary, then the causes may be intermittent and when data is collected to validate the causes, it may show that in fact it is not a cause at all.
  • C&E diagram does not distinguish or check between causes that are both necessary and sufficient. Let’s say X can take two values A and B. A cause is necessary if X=A causes the problem to occur. A cause is sufficient if the problem does not occur unless X = A.
  • C&E diagram does not highlight or show inter-relationship between causes. For example, if there are two causes X1 (possible values A and B) and X2 (possible values C and D) and the problem only occurs if X1 = A and X2 = D but not otherwise, then it is hard to illustrate this on the C&E diagram.
Nevertheless, C&E diagram is a powerful tool that can be used by the team to identify and focus on the most important causes of the problem and you should use it in the initial phases of your project when you are asked to analyze the cause of the problem.

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