Thanks for your research, eyepopslikeamosquito! I like the way you structure your nodes, giving a good overview, and citing relevant literature for deeper understanding. Great work!

I have some experience with 5why, also for software. I think it helps to force teams to think about the ways they develop (software), especially about the interfaces between them. This "side effect" alone gives big benefits (especially because the root-cause is likely related to interfaces).

Regarding the method itself, I have some doubts: too much depends on how exactly you answer the "why" question. Minor differences in wording lead to different directions of the next "why"-question. And thus you might arrive to a totally different root cause.

The inherent assumption of the method ("there is only one (relevant) root cause") is IMHO quite questionable. In my experience, there are often multiple reasons that "come together". Of course you can tackle this by applying 5why to various bones of your Ishikawa-diagram ...

Summary: In my eyes 5why is a valuable tool, but mainly because of the side-effect of triggering discussions and understanding, not because of its simple theory of finding a root cause by asking five questions.

Rata