How many 'Why's' do we answer before we say, 'I don't know, go ask your mother or father!'
Again (to me), the traditional 5-Y tool is technically incapable of expressing multiple paths of logic that occurred simultaneously. It treats failure like it always happens in a linear pattern (never multiple, simultaneous paths).
Like many other veteran analysts, I do not believe there is a magic number of '5' levels deep for our analysis. Why not '3' or '8' levels?
5-Y's also gives the impression there is only 1 root cause. Having been doing this for nearly 30 years, I have never been involved in an investigation that had only 1 root cause. I have never seen a professional investigation of a high visibility event in any industry with one root cause.
We drill down until we understand the flawed reasoning from well-intended decision makers (we call these Latent Root Causes or the flawed management/organizational systems).
I do NOT view 5-Y's as a valid and capable RCA tool for higher severity/priority events that require more comprehensiveness and depth. As always, I am open to debate on this (and I am sure I will hear from opponents), but I will continue to ask for case studies where traditional 5-Y's was used successfully on such high severity/priority events. Perhaps such a case will enlighten us '5-Y' skeptics, as it being a valid RCA tool.
A big part of the problem with these types of discussions on the various forums is, there is no universally accepted and shared RCA dictionary of terms; we all define things in our own way. I think many of us are saying the same thing, but the lack of uniformity of terms makes it appear we are in conflict. We all have unity in purpose though!