5 Why's & Issue Trees

The "5 Why's" is a process of continuing to ask "Why?" at least 5 times, until the root cause of a problem has been found. The method was formerly developed by Sakichi Toyoda, who believed that one had to ask "Why?" at least 5 times before arriving at the real source of a problem.

An example (from Wikipedia):

The vehicle will not start. (the problem)

1. Why? - The battery is dead. (first why)
2. Why? - The alternator is not functioning. (second why)
3. Why? - The alternator belt has broken. (third why)
4. Why? - The alternator belt was well beyond its useful service life and not replaced. (fourth why)
5. Why? - The vehicle was not maintained according to the recommended service schedule. (fifth why, a root cause)

Some problems will have more than 1 root cause. In this case, it is helpful to list all of the potential causes and continue to ask "Why?" for each of them, developing an "Issue Tree" that diagrams all of the potential causes of a problem.

An issue tree starts with the problem on the left and then branches off to the right as we find potential causes of that problem. The tree should have at least 5 generations to the right as you seek to find the real cause of each problem. The tree also allows you to quantify how big an impact each one of the causes is, allowing you to focus on the causes that have the biggest impact on your problem.