What is a Fishbone (Ishikawa) Diagram?
The Fishbone Diagram, also known as the Ishikawa Diagram, is a visual technique for problem-solving invented by Kaoru Ishikawa, a Japanese quality control expert. In manufacturing, the Fishbone Diagram is an effective technique for causal analysis. It aids people in identifying potential causes of a problem and is an especially helpful brainstorming tool when problem-solving is blocked and little quantitative data is available.
How does a Fishbone Diagram work?
The Fishbone Diagram is usually read from left to right and consists of bones, indicating possible causes of a problem, connected to a spine leading into the fish’s head, which symbolizes the defect or problem.
For example, in the diagram below, the shape of a fish’s skeleton is formed by possible causes, grouped by category, for a failed inspection. Here, the causes are categorized by the “5 M’s” in manufacturing: machine, method, material, man/mind power, and measurement/medium. Using these as prompts to generate hypotheses for the root cause of a problem, you write the potential causes under each of these on the “ribs” of the fish. In the image below, for example, the problem is that 12% of the product fails inspection, and one of the potential causes of this related to the materials is that screws were worn.
By visually sorting possible defect causes, identifying cause and effect relationships, and determining which causes are having the greatest impact on the problem, the Fishbone Diagram enables people to address the problem rather than its symptoms. It is frequently used to aid in lean and six-sigma transformations because it allows manufacturers to reduce clutter by identifying the root causes of issues and identify areas for improvement.
The following steps will help you get started using a Fishbone Diagram for root cause analysis on your shop floor:
1. Identify the problem and write it in a box. This is the fish’s head. Draw an arrow leading into the head.
2. Brainstorm categories for potential causes and write them as branches from the arrow.
3. Brainstorm all potential causes and write them under the appropriate category (a cause might fall under more than one category). As you brainstorm, ask “why” each potential cause happens, and use these suggestions to generate more causes.
4. As you group the causes of the problem, it will become apparent which categories are having the largest effect on the problem. When you’ve finished brainstorming, prioritize the causes by how likely they are to be the cause of the problem and how easy they are to fix, focusing on the causes that are having the greatest effect on the problem.
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