Capturing All the Fish in One Net: Cause-and-Effect Diagrams

By Deborah Hopen

In team problem solving, people easily identify potential causes to a problem. But those initial causes identified often do not reflect the true root causes of a problem. Team members may be biased by their experience or eager to jump to solutions so they don’t spend enough time on causation.In team problem solving, people easily identify potential causes to a problem. But those initial causes identified often do not reflect the true root causes of a problem. Team members may be biased by their experience or eager to jump to solutions so they don’t spend enough time on causation.

That’s why Sakichi Toyoda developed the interrogative technique known as 5 Whys. This process encourages team members to delve progressively into deeper causation areas by asking “Why?” five times. As you’ll see, the 5 Whys is an integral part of creating a cause-and-effect diagram, a tool that gives teams both structure and freedom in brainstorming causes. But, first, let’s look at the 5 Whys.

Table 1 provides an example of the sequence of questions and responses used in a 5 Why analysis. In this case, it is quite clear how this approach leads to a different root cause than would have been suspected originally.

Cause-and-effect diagrams are graphic representations of the potential causes of the problem, also called the effect. This tool is used to gather and organize ideas on what causes might lead to a particular problem/effect. Cause-and-effect diagrams also are known as fishbone or Ishikawa diagrams because they look like fish skeletons and were created by Kaoru Ishikawa. Figure 1 below demonstrates how the answers to the 5 Whys are displayed hierarchically on a cause-and-effect diagram.

The cause-and-effect diagram opens up opportunities for all team members to brainstorm ideas and offer possible causes. It shows the relationships between the identified causes and the problem. Through these connections, the final root cause becomes apparent. This information then provides direction to the team as it collects the most important data to prove or disprove its theories about the root cause of the problem.

Steps to Display Brainstormed Suspected Causes on the Fishbone Diagram

1. Create a fishbone structure for the diagram.
2. Write the specific effect generated by the problem in a box on the right side of the fishbone structure.
3. Label each of the major bones with a category name. Common categories, originally referred to as the 6Ms, are people (man), machine, method, measurement, materials and environment (Mother Nature). Another common set of causes is policies, procedures, plant/equipment/space and people. The categories used in a specific cause-and-effect diagram can be quite diverse, but they should be selected in a way that best helps team members organize their thoughts and be able to ascertain similar causes.
4. List each brainstormed cause beneath the bone that categorizes it. Causes may appear on more than one bone.
5. Ask “Why?” five times to identify subordinate causes. Write subordinate causes as increasingly lower-level bones. Note that there will not be five progressively lower levels for every initial cause; in fact, some brainstormed causes actually will be viewed as root causes by team members.
6. Add a footnote to the bottom of the diagram that indicates the date of its creation and the name of one or more participants who can be contacted if additional information about the process used to generate the cause-and-effect diagram or its contents arise. This is a good general practice when creating project documentation.

Tips for Facilitating the Process of Creating Fishbone Diagrams

• Sometimes it is easier to generate two fishbone diagrams during a brainstorming session. The first diagram should capture all the major categories and initially suspected causes. The second diagram can provide more detail on the categories/suspected causes that the team decided to dig more deeply into using the 5 Whys process.
• You can use voting, ranking and evaluation methods to determine which causes the team believes are most important to investigate. A useful tool for prioritizing suspected causes is the cause-and-effect matrix. It helps a team narrow down the field of root causes that need to be validated in the Analyze Phase of a DMAIC project.
• You also can look at the interrelatedness of several causes and gather data to determine if changing one of them leads to changes in another.
• When the 6Ms are used as the categories for the fishbone diagram, it is called a dispersion analysis because the suspected causes shown for each category usually are sources of variation in the effect. Figure 2 below shows a final cause-and-effect diagram that uses this approach. Note that only four of the six categories appear because team members did not identify any suspected causes for the other two categories.

• Another type of fishbone diagram is known as the cause-systems diagram. Its major bones are categories of causes and its minor bones are specific sub-causes related to the specific category.
• The process classification version of the cause-and-effect diagram uses the major steps of the process as its highest-level categories.
• The fishbone diagram can be used for purposes other than organizing and prioritizing causes. For instance, the process classification version provides an easy way to capture brainstorming when developing a plan that involves multiple process steps.

A Coach’s Perspective

When coaching Lean Six Sigma teams, I have observed many occasions where team members think they know the root cause based on experience, rather than a systematic analysis using the 5 Whys process. Generally, this approach does identify symptoms of the problem, but it rarely leads the team to an actual root cause. When’s the last time that you heard a baby cry and guessed the root cause was the parents’ car being in the shop? That’s the trouble with superficial identification of suspected root causes; they don’t go deep enough to prevent recurrence.

At some point, the answer to why actually will be a solution, and that means you’ve gone far enough in your exploration.

Sometimes I am asked whether five is a magic number that inevitably leads to a root cause, and of course, the answer is “No.” The point of the 5 Whys process is to keep asking why until you can go no further. At some point, the answer to why actually will be a solution, and that means you’ve gone far enough in your exploration. So when you ask, “Why were the parents unable to find alternative transportation to the grocery store?” and the answer is, “They had just moved to town and didn’t know who to ask to give them a ride,” you know the solution—make some friends who have cars and some time!

Whenever your team wants to display brainstormed ideas in a hierarchical way that demonstrates their impact on a specific problem/effect or other outcome, the cause-and-effect diagram is a particularly useful tool.

Check Your Understanding

Here are some questions for you to consider regarding the use of cause-and-effect diagrams. They provide a way for you to evaluate your understanding of this tool quickly.

• Why do you think it is important to agree on the level of detail before you start building a cause-and-effect diagram?
• How could a cause-and-effect diagram be used to plan for data collection and analysis
• Using the 6Ms categories, create a cause-and-effect diagram for the effect, “Successful Safety Meeting.” How could this cause-and-effect diagram be used to create a check list?

Deborah Hopen has over 40 years of experience in total quality management, and has served as a senior executive with both Fortune and Inc. 500 companies. She is a Fellow of the American Society for Quality and is the editor of ASQ’s Journal for Quality and Participation.