Using The 5-Whys for Root Cause Analysis

Taiichi Ohno and the 5 WhysWhen a product, machine or component fails; it's important to ask the right questions beyond what the immediate evidence of that failure or problem may indicate. One of the best approaches to this process was developed by Taiichi Ohno (pictured), the engineer and former Executive Vice President who pioneered Toyota Motor Company's production line in the 1950s. Ohno called his method The "5-Whys" and used the example of a welding robot stopping in the middle of its operation to demonstrate the usefulness of his "persistent enquiry" method to finally arrive at the root cause of the problem.

Taiichi Ohno's series of questions were as follows:

1. "Why did the robot stop?"
The circuit has overloaded, causing a fuse to blow.

2. "Why is the circuit overloaded?"
There was insufficient lubrication on the bearings, so they locked up.

3. "Why was there insufficient lubrication on the bearings?"
The oil pump on the robot is not circulating sufficient oil.

4. "Why is the pump not circulating sufficient oil?"
The pump intake is clogged with metal shavings.

5. "Why is the intake clogged with metal shavings?"
Because there is no filter on the pump.

So while the preliminary evidence in the example pointed to an overloaded circuit as the problem; after asking the right questions, it could be determined that the actual root cause was no filter on the oil pump. And while there is no single "fits all" approach for Root Cause Investigations; Taiichi Ohno helped advance the thinking of the generations of engineers, quality assurance and material testing professionals who have followed by putting the concept of "persistent enquiry" into an easy-to-understand methodology.

Credit: This article was written using information from various sources, including Toyota Motor Company.

Diagramming Potential Factors

The "5-Whys" give us a simple way to determine the relationship between different root causes of a problem without statistical analysis. A Fishbone or Ishikawa Diagram can be used in conjunction with the "5-Whys" Method to identify and group potential factors causing an overall effect into various categories. See below for an example of an Ishikawa Diagram, named for its creator, Kaoru Ishikawa.

Ishikawa Diagram

Diagram Source: Daniel Penfield via Wikimedia Commons

Why choose ATRONA Test Labs for your next Root Cause Investigation?

At ATRONA Test Labs, we're comprehensive root cause investigation and formal failure analysis specialists with over 2,000 completed assignments. We always strive to determine the exact root cause of the failure no matter how big or small. Our approach and analysis, team, and reporting style sets us apart from other labs. Our customers like that about us and they appreciate the support and insight we provide during and after the project is completed. And because we have a state-of-the-art, multi-lab facility with all of the tools needed for any failure analysis or root cause investigation, we are able to maintain all-important control over your samples and the testing procedures without outsourcing. Like to learn more, including how to preserve your samples and ship to us for testing? See Root Cause Analysis at ATRONA →