Fishbone Diagram / Cause and Effect Diagram

The purpose of a cause and effect analysis is to identify the causes, factors, or sources of variation that lead to a specific event, result, or defect in a product or process. A fishbone diagram is simply a tool to be used along with Brainstorming and the 5 Whys.

In a fishbone diagram, the various causes are grouped into categories and the arrows in the image below indicate how the causes cascade or flow toward the end effect. Due to its simplicity, the diagram is often drawn on a white board during a brainstorm session. I designed the above template so that it would easy for someone familiar with Excel to use during a meeting to record the ideas as they are discussed.

Steps to Using a Cause and Effect Diagram

1. Define the Effect: Be specific.
2. Choose Categories: The fishbone diagram template is set up with the most common set of categories, but you can add or remove categories based on your specific case. See the example categories below.
3. Brainstorm Possible Causes: Using the fishbone diagram while brainstorming can both broaden and focus your thinking as you consider the various categories in turn.
4. Ask Why?: You really want to find the root causes, and one way to help do that is to use the 5 Whys technique: asking "Why?" or "Why else?" over and over until you come up with possible root causes. "Improper handling" is not a root cause, while "Failing to wear Latex gloves" might be closer to a root cause. But, you could still ask "Why was he/she not wearing gloves?" with the possible response "There were none available." It is a lot easier to take action against the inventory problem than just the generic "improper handling".
5. Investigate: Now that you've come up with possible causes, it is time to go gather data to confirm which causes are real or not.

Common Categories in a Fishbone Diagram

During a brainstorm session, a fishbone diagram is usually used very loosely, meaning that sometimes branches (what I have labeled as primary and secondary causes in the fishbone diagram below) may actually represent sub-categories of causes rather than actual causality.

When a cause and effect diagram is used to represent causality, then the primary and secondary branches taken on very specific meanings:

When a fishbone diagram is used for simply categorizing possible causes, then instead of listing Dropping in the place of a primary cause, it might be listed under the sub-category Improper Handling, with Dropping and Throwing as different causes that fit under that sub-category.

A tree diagram, probability tree, or root cause analysis is geared more towards thinking in terms of causality, while using a fishbone diagram tends to make people think in terms of categorization. Using the fish bone diagram loosely may result in a combination of the two approaches as the group oscillates between categorizing different causes and asking "Why?" or "Why else?".

Although I've never seen any reference for this technique, I use the following rule to distinguish between categorization vs. causality when using a fishbone diagram:

Just as the main categories (Equipment, People, etc.) are highlighted by placing a circle or box around them, if you include sub-categories in your cause-and-effect diagram, circle the sub-category so you can distinguish between categorization vs. causality.

The following tree diagram shows what the cause-and-effect diagram would look like if you distinguish between categorization (grouping of causes) and causality.