Five why's states that if I have to ask "Why?" more than 5 times to get at why something is the way it is, it needs to be simplified, in the sense that lack of simplicity/excessive cognitive load is in and of itself, a form of defect.
Because it isn't. 5 whys is for exploring the neighborhood of the problem without going deep down a rabbit hole. It's like timeboxing an activity or putting a bounds on a search algorithm. The objective is to find something near enough to act on and partially [0] address the systemic causes, but far enough to not just be the proximate cause. "Battery was dead" is a proximate cause, many people stop at proximate causes. 5 whys forces you beyond that.
It should also be combined with other techniques that elaborate, more properly, on the model of the system in which the problem occurred. There you will find the deeper systemic causes and can start addressing and mitigating the issues. But that will take a lot longer to explore and resolve than the 5 whys answer of "Get a proper tuneup every 5-10k miles".
[0] Ideally fully, but 5 whys is an expedient, non-thorough technique so that's a reach.
No, you have it exactly backwards. Five whys is explicitly depth first search. It was originally employed by Toyota Motors, and the key to actually getting at what the actual causes of issues were. This allowed Japan to avoid entire classes of defect due to their acceptance and implementations of the teachings of W. Edwards Demmings. It never made the leap to the United States, because doing things right is such a foreign concept over here, where management would gasp at the concept of a single worker shutting down the entire factory upon detecting a problem, whereas in Japan, the andon was key to getting issues that would compromise the Quality of the end product addressed.
It's:
I have a problem,
Why? Because of this other thing.
Why? What causes other thing?
The other thing is dependent on yet another thing and that thing.
Are those things a problem? If so, why?
Iterate until done.
The "Five" aspect came into play in that after a certain number of "Why's", it was often the case that the fact it took so many Why's to get to the root cause, that the complexity itself constituted it's own type of defect.
I might need to revisit the book I first read of it in, believe it was titled "Elegance" or some such.
EDIT: Found it.
In Pursuit of Elegance: Why the Best Ideas Have Something Missing by Matthew E. May
EDIT2: Nope, false alarm. Drat it, I will find that reference.
It's in this library somewhere... In the meantime, forget it; maybe I'm hallucinating it, because clearly I haven't refreshed it in a while, and the old wetware has taken a hell of a beating over the last couple years.
I didn't say it wasn't depth first so I have no idea what you're responding to with this. Can you point out where in my comment I said it wasn't depth first?
I just said it's bounded, because it is. Even the link you've posted states that "why?" is only asked 5 times. But even if you take the 5 as suggestive rather than a strict limit, giving a limit (even a soft one) implies a boundary to the search. If it was meant to be unbounded it would just be called "Whys". 5 pushes you beyond the proximate cause and towards the actual root cause (of course, "root cause" is itself a poor phrase as it implies a singular cause that can be addressed to prevent the problem from reoccurring), but then the root cause could be 10 or 20 or 30 whys deep. So unless you go beyond the point suggested by the name and publications about it then you won't find it.
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EDIT: I see you edited your comment after I hit reply and didn't mark it as such, good form.
Anyways, about it being depth first I'd actually disagree. Even if you believe that it should be taken as an unbounded search (which I also disagree with, obviously, because there are better tools than 5 whys for such a major task) it should not be an unbounded depth first search. That would be a form of malpractice.
The more accurate description would be something like iterative deepening. You go down a few layers with something approximating a depth first search, and then you back track and try the other branches. If you go down one rabbit hole arbitrarily deep you may or may not find a causal factor but you will have missed many others along the branches you chose to ignore for the sake of your unbounded depth first search (again, a form of malpractice).
