One of the problem solving steps is the root cause definition. An alternative to this phase analysis is the double Ishikawa diagram, which is performed for two scenarios: why the problem happened and why it was not detected.
The above suggestion should most often be used in two scenarios. First, if there is an issue that affects safety and / or regulation. Secondly, if the problem concerns a large affected population of components sent to the customer.
Document structure
The document itself looks familiar, but it’s a little different from stadard approach. I first encountered this type of diagram while working as an OEM supplier and I regret very much that I didn’t know this tool earlier. Why? Because it gives us the opportunity to look at the problem from a different perspective, focusing on two areas closely related to Occurrence and Detection from PFMEA, that is:
– Why problem occurs?
– Why problem undetected?
Figure 1. Double Ishikawa Diagram – Application Example for Customer Complaints
Thanks to this approach, we are able to more accurately identify all the causes that influenced the production of the defective product. Most often, as in life, we deal with a coincidence of several events that led to the problem. Of course, in this case, we should apply appropriate prioritization – for example using the 80/20 rule. In practice, the team should address up to three root causes.
Double Ishikawa Diagram – Benefits
The director of the facility where I work used to say to the problem solving team (not only for quality issues) , “Show me your Ishikawa.” Why? Because by seeing how many factors the team had identified, they could see how much effort had been made by them and how much time had been spent on the analysis.
He didn’tt expect that 5 x Why would be defined for each identified cause. He was interested in the number of areas that were examined. I encourage you to implement this approach at your facility as well. You will see how you can be positively surprised when it becomes your daily routine.
You can download an automatic, editable Excel form for free on the Free Quality Tools
Document name: Double Ishikawa for Why Happened and Why Undetected – Excel form
Dariusz Kowalczyk
