From Toyota's production system, credited to Sakichi Toyoda and first set down by Taiichi Ohno (Toyota Production System, 1978).
Asks why a problem happened, then asks why again of each answer, producing a short chain that ends at a cause in the process worth fixing.
When to reach for it
After a defect, outage, or repeated operational failure, when the obvious explanation looks like a symptom of something further upstream.
How it runs
- State the problem in concrete, observable terms.
- Ask why it happened and record the direct cause.
- Ask why of that cause, and keep recording each level.
- Stop when the answer points to how the process or system is built.
- Fix that cause and check whether the problem returns.
Where it sits in SOLVE eX
Go to the source
- Toyota Production System: Beyond Large-Scale Production, Japanese edition 1978; English edition, Productivity Press, 1988, 1978
- The Toyota Way, 2004