TUESDAY, NOVEMBER 13, 2018
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Six Sigma Tools & Templates Cause & Effect The Fundamentals of Cause-and-effect (aka Fishbone) Diagrams

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

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

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

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” at http://www.isixsigma.com/implementation/sports/case-study-applying-six-sigma-to-cricket/ (Click to enlarge)

Figure 4: Example of Completed Fishbone Diagram – graphic excerpted from “Case Study: Applying Six Sigma to Cricket” at http://www.isixsigma.com/implementation/sports/case-study-applying-six-sigma-to-cricket/ (Click to enlarge)

To learn more about cause-and-effect analysis and related topics, refer to the following articles, discussions and blogs on iSixSigma.com:

Additional resources are available for purchase on the iSixSigma Marketplace:

For more cause-and-effect articles, click here.

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Comments

caseyem

It’s not always possible to get to the “true” root cause. For example, a hole in a wall may be due to a leaky pipe. A leaky pipe maybe due to the material used. The material’s fragility is due to physical properties. It’s physical properties are due to ….? When have you reached the “true” root cause?

But often we just want to mitigate the effect, so we don’t need to get to root causes.

I’ve never seen anyone teach/explain when to stop digging deeper, i.e., when to stop asking why. The advice I give is when by the answer you know what action to take to mitigate the effect. Thus, a less knowledgeable and capable person will ask more whys than a more knowledgeable and capable one.

A third issue is that once a list of potential causes is generated, little is said about how to reduce the list without subjectivity. I was happy to see you state: “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.”

Fourthly, it’s easy to give examples of the Five Whys. But I have never seen anyone explain how to get the answers, giving the impression that we always know the answers. If we know, then why not just solve the problem?

Finally, because brainstorming’s purpose is quantity over quality, I prefer using another set of categories that keeps the list to what is relevant: process steps. Rather using the 6 Ms or its variants, I list the process steps. Of course, once I decide to use process steps, then I no longer need to use a fish bone. An example of a template for process steps is the FMEA or variations of it. Using the FMEA, I then replace Potential Failure Mode with Actual Defect.

Terence Winslow

Powerful Presentation Diagrams!!! Laser precision for “GETTING IT DONE” Teams & Personal Development Masters…

Pranay Kumar

Good article. Fishbone is an imperative graphical representation of the root causes identified. However it is THE most challenging excercise in any consulting/improvement assignment.

C&E is the defining moment to the improvement jourrney,

Yuvraj Hedaoo

Fish bone is powerful tool to understand the root cause and at the same time, challenging task. Further, we can leverage this diagram to validate the real cause of the problem.

Olive

Very good thank you



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