Five Whys

Five Whys is the root-cause-analysis technique developed at Toyota that asks why, and then why again, until the answer stops describing a symptom and starts describing a system. The rule of thumb is that five iterations are enough to cross from the visible failure to the process or management cause underneath it, and the discipline is to keep asking past the first plausible answer, because the first why almost always names a human error and the fifth why names the system that let the human error matter. It is the lightest RCA tool and the fastest, and a team that cannot run a Five Whys cannot run a heavier method.

The limit of Five Whys is the single-fault path. It traces one chain of cause and effect to its root, and it breaks down on a problem where several causes interact — a defect with a material cause and a method cause and a measurement cause, each necessary, none sufficient. For that problem the fishbone is the better tool, because it maps the cause space before the team drills a chain. The other failure mode is the why that stops at training — a human did not follow the procedure, so train the human — when the root cause is the procedure itself or the design that made the procedure impossible to follow. See root-cause analysis and fishbone.