(a.k.a. “5 Why’s & 1 How”) A pen and paper tool for discovering the root cause of a problem or issue.
(1st Why) Why is there oil on the floor?
(Answer after investigation) It is leaking from a hose.
(2nd Why) Why is oil leaking from this hose?
(Answer after investigation) The hose is rubbing on a fan belt.
(3rd Why) Why is it rubbing on the fan belt?
(Answer after investigation) The fan belt housing is very loose.
(4th Why) Why is the fan belt housing so loose?
(Answer after investigation) Some of the bolts that hold it in place are missing or broken off.
(1 How) How do we fix the problem?
(Answer) Replace the hose and fan belt, remove broken bolts on fan belt housing and replace with new ones, torque all bolts to proper specifications, check bolt torque with regular preventative maintenance.
Note: You probably noticed our example only includes 4 whys. The number of whys is much less important than finding and fixing the root cause of the problem you are having.