Five Whys

Origin. Toyoda Sakichi; the Toyota Production System.

Mechanism. Iterated causal regression. Each answer becomes the next question's subject, walking backward along a causal chain until it reaches a node that is under the organization's control.

Procedure. State the failure. Ask why. Ask why of the answer. Five is nominal — the stopping condition is reaching an actionable cause, not reaching the fifth iteration.

Applies to. Single-cause mechanical and process failures, which is what it was built for.

Limitations. Assumes a chain where there is a lattice. It selects one path through a causal graph and calls it the cause; run twice by different people it produces different roots. It also terminates at 'human error' unless explicitly forbidden to.