Five Whys
Also known as: 5 Whys
Iteratively ask 'why' to drill from symptom to root cause, originating in Toyota's TPS.
The Five Whys is a root-cause analysis technique developed by Sakichi Toyoda at Toyota and operationalized within the Toyota Production System: when a problem is identified, the analyst asks 'why?' five times in sequence, treating each answer as the question for the next round, until reaching what is plausibly the root cause rather than a contributing symptom. Taiichi Ohno emphasized that the method's purpose is to reveal that 'the root cause of any problem lies hidden, further away from the source' than the immediate symptom suggests. The five-iteration count is a heuristic rather than a fixed rule — sometimes three iterations suffice, sometimes more are needed. The technique is one of the basic tools of Lean and Six Sigma problem-solving, often paired with the Fishbone Diagram for category-based brainstorming, and is widely used in incident postmortems including software engineering and healthcare.
Core components
- Sequential 'why' questioning
- Each answer becomes next question
- Five iterations as heuristic count
- Distinction between symptom and root cause
- Often paired with Fishbone Diagram
- Foundation for countermeasure design
- Toyota Production System origin
Primary use case
Root-cause analysis in operations, manufacturing, software incident response, and healthcare; component of A3 problem-solving; teaching tool for systematic causal reasoning; quality-improvement initiatives.
Common criticisms
- Linear chain of single causes oversimplifies multi-causal real-world problems
- outcome depends heavily on which 'why' is asked at each step — different paths yield different root causes
- weak at addressing systemic, organizational, or cultural causes that don't sit on a single causal chain
- can converge on whatever the analyst already suspected
- doesn't surface contributing factors that aren't on the chosen path
- five iterations is heuristic and can be too few or too many
- tendency to attribute causes to individuals rather than systems
- better as a starting point than a complete RCA method.
Lineage
- Child of
- Toyota Production System
- Siblings
- Fishbone Diagram, Pareto Analysis, After Action Review
- Derived from
- Toyota Production System