The Fundamentals of Cause-and-effect (aka Fishbone) Diagrams

A popular means for identifying the causes of a particular problem, or effect, is the aptly named cause-and-effect diagram. As the completed graphic resembles the bones of a fish, it is also commonly referred to as a “fishbone” diagram (Figure 1).

Figure 1: A Fishbone Diagram

The head of the “fish” is the problem statement, such as “late delivery of product XYZ” (Figure 2). From the head is drawn a straight horizontal line, and all of the “bones” coming off that line represent categories of possible causes. For example, many practitioners use the 6Ms as the categories for the bones:

1. Man (personnel)

2. Machine

3. Methods

4. Materials

5. Measurements

6. Mother Nature (environment)

Figure 2: Filling in the Bones

However, what set of categories is used for a given analysis can be modified to fit the situation.

Team members – which should include subject matter experts and those who work within the processes related to the problem – then brainstorm the causes of the defined problem. For example, a possible cause of late deliveries of product XYZ within the Machine category could be “frequent line stoppages.”

Another approach is to start by brainstorming the possible causes of the problem, and then determining appropriate categories based on what causes were identified. Sticky notes are particularly useful for this method – write one cause per sticky note and they are easily moved for grouping.

A team is likely to find that once they have identified possible causes, they need to delve a little further to find a true root cause. For instance, as shown in Figure 3, each cause coming off the main category “bone” could have one or more lines with sub-causes coming off of it. Application of the 5 Whys at this point can help drive the team to the root cause. Why are there frequent line stoppages? Because the material jams. Why does the material jam? Because it is out of spec. Why is it out of spec? Etc.

Figure 3: Determining the Causes

Teams that start brainstorming within defined categories may find that as they dig further into a cause, the sub- or root cause might better fit into a different category (e.g., out-of-spec material that causes jams more appropriately fits in the Material category, rather than Machine). Not to worry. The categories are generally used to help spur ideas and should not constrain a team with unnecessary boundaries.

While the cause-and-effect diagram has the benefit of being a visual tool that utilizes the input of many team members, its drawback is that it is based on perception and does not constitute a quantitative analysis. For that reason, it is best suited for projects in which hard data is unavailable, or as preliminary work to identify potential causes worthy of data collection and further analysis.

After the diagramming is complete, the improvement team can tackle the root causes – either by immediately addressing the identified cause, or by using the information as input for additional analysis as needed.

Figure 4: Example of Completed Fishbone Diagram – graphic excerpted from “Case Study: Applying Six Sigma to Cricket”