Five Why Method of Six Sigma

The five why method was initially used by Toyota Motor Corporation for root causes analysis of various manufacturing related problems. Various tools are available for root cause analysis but because of simplicity and effectiveness five why method becomes part of many methodologies like Kaizen and Six Sigma.

• Identify the problem for which root cause to be find out.
• Ask “Why the problem has happened?” you will get an answer (or rather another problem).
• Take the first answer (or problem) and again ask “Why?” You will get second level answer.
• Do such five iterations and you will get the root cause of the problem.

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.

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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.

The iterations of “Why?” can go further beyond five but in Toyota training manual it is postulated as five why method is sufficient to get root cause of any practical problem Five why method is used in “Analyze” phase of a six sigma project. The success of five why method depends upon the co-operation and flexibility of the mangers of the organization.