The 5 Whys traces a symptom back to its cause by asking why repeatedly, each answer becoming the subject of the next question. Five is a rule of thumb rather than a count — you stop when you reach something you can act on.
Its value is that it is fast enough to run in the room where the problem was noticed, with no preparation and no tooling. That also bounds it: it follows a single chain, so a problem with several contributing causes will be answered with whichever one the group thought of first. Where causes branch, an Ishikawa diagram covers the ground properly.
The chain has to be checked backwards as well as forwards. Read from the bottom up, each cause should plausibly produce the one above it; if it does not, the chain drifted into narrative somewhere and the remaining answers are speculation.
It goes wrong when the answers name people. “Because the operator forgot” ends the analysis at the point where it should start — what made forgetting possible, and what allowed the omission to pass undetected, are the questions that lead anywhere useful.
