A Cause and Effect Diagram is a pictorial diagram showing possible causes (process inputs) for a given effect (process outputs) It is also referred to as the “Ishikawa diagram” or “fishbone diagram.
Created by Kaoru Ishikawa (1968) that show the causes of a specific event. Common uses of the Ishikawa diagram are product design and quality defect prevention, to identify potential factors causing an overall effect.
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)
- 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
Fishbone Best Practices
- Focus on cause, not solutions.
- Go beyond the superficial levels to find root causes. Don’t merely outline options.
- Ask “why” until it is absurd to continue.
Fishbone Diagram video
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
Answer: A fishbone diagram. A fishbone diagram will help in the root cause analysis to identify potential factors causing the greatest overall effect on the time between actions.
A scatter diagram, box and whisker plot, and histogram are all great visual management tools that will represent data that was recorded. However, the question did not indicate that data has been recorded yet.