Lean Six Sigma: Getting to the root cause of the problem
// 2020 Feb 12
By Regu Regupathy, Master Black Belt, Rutgers University
When something goes wrong, we want to identify the cause(s) of this outcome. We brainstorm, based on our experience/perception as to what caused the outcome.
Let’s say your son brought home a poor report card. You and your spouse analyze the reasons why your son’s grades are poor. In addition to your own thoughts, you may engage your son and the teacher in the brainstorming process to identify the causes.
The process goes something like this:
1.Identify the problem (the effect or outcome)
2.Brainstorm and list possible causes
3.Identify 5 to 9 of the most probable causes to work on
In a formal setting, in a lean process improvement practice, you will use an Ishikawa Diagram, a.k.a. a fishbone diagram, like the one shown here.
The major “bones’” titles shown are ones commonly used to trigger causes in each of these titles, but you can change these to reflect the factors suitable for your process.
Related to your son’s report card, the head of the fish (Effect) would be poor grades. You, your spouse, your son, and the teacher will each have ideas based on personal experience and perception of what the causes are. To be successful, document all ideas shared by the team. In other words, list all possible causes without challenging any. After all ideas are exhausted, the participants may be asked to select, by voting, the most probable causes.
Take a look at the fishbone diagram, shown below, which contains possible and probable causes. The probable causes are hilighted in yellow.
Should we attempt to address all probable causes to improve the grades? Probably not. Note that all the causes listed in the diagram are based on perceptions and gut feeling. They need to be analyzed with data or investigated further to ensure that they are valid and have significant influence on the grades. This will result in the identification of 2 to 5 critical causes which needs to be addressed in the improvement phase of the project. More on these steps in future blogs-stay tuned!
Could you choose other titles as major bones? Absolutely! For example, in your organization and process, if the cycle time is too long and the process involves four major departments, you may use these four departments as major bones. Or any number of major bones appropriate to the process under consideration.
A couple of notes:
- Don’t waste too much time arguing about which bone a particular cause belongs. It does not matter as long as that cause is listed somewhere in the diagram.
- Make sure the causes listed are actionable. For example, if “scoring method” is listed it does not convey what action is needed. An actionable cause such as “scoring method not understood” conveys what needs to be improved-the student’s understanding of the scoring method.