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Basic Management Philosophy Handbook for Toyota/G.M.

Asking "why" five times is one of the ways to get closer to the real 'cause'. We should learn more from very young children. Listen to them … "Why … why … why"? They want to understand! 

Example … A pump burned out - why? There were metal filings in the bearings - why? The filter screen wasn't on the filter - why? Because it fell off and was at the bottom of the coolant basin - why? The maintenance man didn't put a lock nut on the filter. Why? … (problem defined).

Five "whys" will get you close to solving the problem; one "why" will help you react to the symptoms.