A Cause and Effect Diagram (aka Ishikawa, Fishbone) is a pictorial diagram showing possible causes (process inputs) for a given effect (process outputs). In other words, it is a visual representation used to find out the cause(s) of a specific problem.
The Cause and Effect diagram is also referred to as the “Ishikawa diagram” or “fishbone diagram. The design of a diagram looks much like a skeleton of a fish. Fishbone diagram typically draw right to left, with each large bone of fish branching out to include smaller bones containing more detail.
The Cause and Effect Diagram (aka Ishikawa, Fishbone) introduced by Kaoru Ishikawa (1968) that show the causes of a specific event. He also introduced the company-wide quality control (CWQC) and also Quality circles concept in Japan. Common uses of the Ishikawa diagram are product design and quality defect prevention, to identify potential factors causing an overall effect. It is one of the seven basic tools of quality control.
When to use the Cause and Effect Diagram
- When the problem has multiple probable causes
- To identify the possible root causes for an effect
- Identify and sort interaction among the factors on an effect
- To initiate appropriate corrective action for existing problems
Why use Cause and Effect Diagram
- It is a basic step to study a problem/issue to determine the root cause
- To study all the probable causes of why a process is beginning to have a problem or breakdowns
- Need to identify areas for data collection for further study
How to develop a Cause and Effect Diagram
The Cause and effect diagram is a structured graphical diagram to list all probable causes and also their relationship with the main effect or problem. The diagram’s basic input is the brainstorming that pushes to identify all the possible causes rather than just more noticeable ones.
Following are the steps to draw fishbone diagram
Step 1: Identify the problem:
- Identify and clearly write down the effect or a problem that needs to be analyzed
- Create an operational definition of a problem to ensure all the stakeholders will understand it
- An effect may be positive like objective (to improve the process further) or negative like a problem (to identify customer dissatisfaction).
Step 2: Determine Effect or problem:
- Draw a horizontal line, this is a spine
- At the end of the spine, towards right draw a box and determine the description of an effect
Step 3: Identify major causes contributing to the effect or a problem
- Establish main causes and write down few main causes on the above horizontal line and few on below horizontal line and are placed on the tail of associated arrows.
- Each cause or reason for imperfection is a source of variation. Causes are usually grouped into major categories to identify these sources of variation. The categories typically include: (Also see the 5Ms and 1P)
- Man/People: Anyone involved with the process
- Methods: How the process is performed and the specific requirements for doing it, such as policies, procedures, rules, regulations and laws
- Machines: Any equipment, computers, tools, etc. required to accomplish the job
- Materials: Raw materials, parts, pens, paper, etc. used to produce the final product
- Measurements: Data generated from the process that are used to evaluate its quality
- Environment: The conditions, such as location, time, temperature, and culture in which the process operates
Step 4: Identify sub causes
- Identify as many sub causes that are relevant to the main causes and are placed under each main cause category
- For instance, if the sub cause is applicable in multiple places, list down under each main category
Step 5: Analyze the diagram
- Now the fishbone diagram shows all the possible causes of an effect
- Identify causes which are not having an impact on effect and also identify causes that need further investigation
- Further, perform 5Why analysis to the identified causes (or also called noises) to arrive actual root cause.
Cause and Effect Diagram Example
Fishbone diagrams are used to identify the root causes of a problem in the “Analyze” phase of Six Sigma’s DMAIC (Define, Measure, Analyze, Improve, and Control).
Example: XYZ is a valve manufacturing unit receiving persistent customer complaints about the valve diameter. The General Manager of the plant asked QA and Production team to conduct root cause analysis. So, the team performed Brainstorming and draw the Cause and Effect diagram to identify all the possible causes.
Team identified the wrong calliper and wrong procedures are the probable causes. Further, the team has to perform 5Why analysis to identify the root cause for these probable causes.
Benefits of Cause and Effect Diagram
- Helps to identify all the probable causes of an effect to identify root causes further
- Further, it graphically displays all the possible causes related to a problem to discover root causes
- Fosters teamwork – inspires the team to brainstorm until the elimination of root cause
- Easy to implement
- A common understanding of factors causing the problem
- Establish brainstorming relationship
Fishbone Diagram Best Practices
- Focus on the cause, not just solutions.
- Go beyond the superficial levels to find root causes. So, don’t merely outline options.
- Ask “why” until it is absurd to continue.
Cause and Effect Diagram Template
Fishbone Diagram videos
Further, Minitab put out a good video on how to use their software to brainstorm and create a fishbone diagram. If you’ve never done this before, this is a great reference.
ASQ Six Sigma Black Belt Exam Fishbone Diagram Questions
(Taken from ASQ sample Black Belt exam.)
Question: A company’s accounts payable department is trying to reduce the time between receipt and payment of invoices. If the team has just completed a flow chart of the process and identified the critical steps, which of he following tools should be used next?
A) Fishbone diagram
B) Scatter diagram
C) Box and whisker plot