Find Your Root Cause with Cause and Effect Diagrams
A cause and effect diagram is a tool used to facilitate root cause analysis for a defined problem. The diagram provides a structured way to record potential root causes during brainstorming, encouraging teams to think about a problem systematically and to dig deeper to discover less obvious causes.
The cause and effect diagram also referred to as an Ishikawa diagram after Dr. Kaoru Ishikawa, the Japanese quality control expert who invented it. The popular name, “fishbone” stems from the fact that the diagram resembles the skeleton of a fish.
The analysis starts with a problem to be investigated. This problem is written in the form of a question on the right side of the page. An arrow, or sometimes a drawing of a fish head, will point to the question under consideration. To the left of the problem statement, a horizontal line divides the paper in two. This is the “backbone” of the cause and effect diagram.
The next set of bones represents major categories of factors that could contribute to the root cause. These category names are written along the top and the bottom of the paper. Angled arrows point back to the backbone and toward the head, forming a herringbone pattern.
There are many methods that have been used in the past to obtain categories for different types of problems. For example, the manufacturing industry has used the 6 M’s. Whereas service and administrative problems use the 8 P’s to help determine a starting point. In addition, the Service industry may also use the 4 S’s to categorize problems.
Once the basic skeleton is in place, the analysis begins by listing factors that contribute to each category of issues that produce the root cause. These are written above arrows that point back to the category line. These lines in turn can have their own lines pointing into them, breaking down factors that contribute to it. This can go on infinitely, but will be difficult to draw beyond a few levels for obvious reasons.
The reasons which causes the end result is analyzed by a team through brainstorming each category, using the skeleton of the diagram. With reference to the context of each category, a question is formed according to the problem and asked the team members to answer them. Generally questions like “why this is happening” is asked and according to the category the question varies to “How do factors influence this category?” The brainstorming process ends when team members have no more useful items to add to the diagram. The most likely root causes are identified after a careful analysis. If the same issue occurs within multiple categories, it is likely that the issue is an important root cause of the problem. Similarly, densely populated areas within the diagram hint at areas of significance.
