Root cause analysis (RCA) is a powerful way to identify systemic causes of manufacturing problems and to prevent the same issues from recurring. But too often the methods fail to achieve the results you expect—usually because you’ve chosen the wrong tools for the job.
There are many root cause analysis tools to choose from, but none as powerful as the flowchart and the logic tree, notes quality management pro Duke Okes in his Audio Solutionz webinar “The Two Most Important Root Cause Analysis Tools—and How to Use Them.” In the presentation, Okes explains why the cause-and-effect diagram is not useful for complex systems and outlines 6 ways each to use flowcharts and logic trees, as well as how to use these tools interactively.
Build a Logic Tree: Move from Definition to Illustration
One of the most powerful root cause analysis tools is the logic tree, which can answer two kinds of questions, explains 3M: “why” and “how.”
“The clearer and more specific your question, the better it will work in your logic tree,” the company notes.
Here is how to build a logic tree of your own:
- Define the problem.
- Offer possible answers—at least two.
- Suggest the most important parts for each solution.
- Illustrate how to make each solution a reality.
“Remember, not every piece of your logic tree will have information for every step,” 3M adds. “Some levels of the logic tree end quickly, while others extend much further.”
Expect 5 Possible RCA Outcomes
To capture more detail in your logic tree, start with a “failure event” and move up to “failure mode” (the possible cause of the event), Meridium suggests. Be sure to offer a hypothesis that might explain the problem.
Note: Your hypothesis can have five possible outcomes, Meridium adds:
- not true
- human cause
- latent cause
- physical cause
A latent cause of a problem often stems to inadequate accountability—i.e., flaws in the system related to poor communication, incorrect data, bad decisions, or failure to follow recommended safety guidelines.
Investigate with Flowcharts: Use Yes/No Questions
A second useful root cause analysis tool is the flowchart. The process to investigate with a flowchart is somewhat simpler, explains Lucidchart: When you diagram your problem with this method, each step hinges on a yes/no question.
Each step in the chart (often called a “node”) poses a question. Or, as Concept Draw explains it, each node represents a “‘test’ on an attribute (e.g., whether a coin flip comes up heads or tails).” Then each branch coming from a node represents the outcome of that question or test.
The idea behind the flowchart, explains a Community Health Plan of Washington RCA report, is to answer the critical questions listed below—and to avoid hindsight bias in answering them:
- What happened (or is still happening)?
- How/why did it happen?
- How can you prevent it from happening again?
Indeed, the end goal of a root cause analysis, explains Okes in his webinar, is to generate a more powerful tool for investigating how a system, process, or device failed—and to do so by using a sequential and structural process which can quickly eliminate a large number of potential causes to focus on likely suspects. Master this, he says, and you are well on your way to more successful quality management.