Many thanks to all for your responses. A particular thanks to What Limits for providing the specific example. My view is that Tap Root is quite useful especially for trouble shooting equipment failures.
My background is that of an ex military pilot who now works with the petrochemical industry. I still fly privately but have no commercial experience. A good proportion of my work involves RCA analysis of 'events' and advising on how to fill in the holes in the Swiss cheese model. I study aircraft accidents as they are some of the most investigated accidents there are.
In essence, all RCA techniques are based on the 5 'Whys'. Why Tree, Why Root, Drill Down etc all use the same basic techniques. Some require a skilled facilitator (me
) to keep things focused.
My view is that you can now go down to 7 'whys' if necessary.
I also define 'root cause': 'The lowest level system failure that - if fixed - would have prevented the event'.
I agree that there can - sometimes - be more than one of these. There is, more often, multiple causal factors, contributing factors; call them what you wish. However, if you were to accept my definition, you would probably arrive at only one root cause and multiple CFs.
I like to split the analysis into three stages:
1. The 'What Tree': this analyses what happened.
2. The 'Who Tree': this gets to who dis what and when. Often difficult because of defensive attitudes and turf wars.
3.Keep asking 'why' and, with skill, you get to the 'why' at a systems level. This is the bit you fix.
Usually that single factor is the action sequence of the crew immediately prior to impact.
This is at the 'who' level. If you were to ask yourself why the Captain made inappropriate control inputs you will identify a number of CFs. You need to get past the actual actions to the systems level failure. Start from the POV that most people try to do the right thing.
Apollo 30:
Last weekend I worked throug the Colgan Air Accident report (NTSB) and there I really experienced that there was no root cause. There were so many important fragments, but no single root cause.
I have read the report. What? 40 plus recommendations. The Probable Cause (not root cause) was the inappropriate reaction to the stick shaker.There are 4 CFs listed.
What is also clear in the report is the NTSB's total frustration with the failure of the FAA to implement earlier recommendations. Indeed, if you were to look a deeper root causes, this might be one!
However, based on my earlier definition, you could have the root cause of this accident being Colgan's failure to ensure that pilots had the requisite skills to do the job. This might look at - in this case - the hiring process.
To me the crux of any RCA has to be understanding the Problem Statement or Definition to start with.
Totally agree. Get that wrong and you don't stand a chance!!!! You have to analyse the right thing or try to solve the right problem! Many examples of that including the NASA pen (fairy) tale.
Thanks. Interesting discussion.