Who caused the accident isn't a problem statement. A driver hit a pedestrian is. Sure, from there maybe you have some weird bias against pedestrians, but in most cases five whys would say the root cause is driver inattention. Poor framing of the initial problem is as much an issue with the method as finding bad answers to "why".
Right, and "root cause" only has meaning in the context of some goal being achieved. If you're a detective investigating the scene of the accident to determine culpability, your goal is very different than if you're a civil engineer investigating the accident for ways to improve traffic management. It'd be really strange for either of them to conclude their report with a recommendation to increase World Health Organization's budget for alcohol abuse treatment.
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Therefore, Your Honor, my client is innocent. Why has this tragic accident occurred? I shall tell you to look no further than the root cause...
