An Example of How to Use Five Why Method
Problem: Spitted oil on machine shop floor.
First Why: Why oil is spitted over shop floor?
Answer: Because one of the CNC machine has leak in its oil tank.
Second Why: Why the CNC has leak in its oil tank?
Answer: O-ring of one port of tank has spoiled.
Third Why: Why the o-ring has spoiled?
Answer: The rubber (o-ring material) quality is poor.
Fourth Why: Why we are using poor material quality O-ring?
Answer: It is cheapest.
Fifth Why: Why we are using cheapest O-ring?
Answer: Cost is the main supplier selection criteria.
So the supplier selection criterion is the root cause of the oil spitting problem. As short term action the O-ring needs to be replaced and the spitted area to be cleaned but in long term the supplier selection criteria need to be modified.