42 first principles

5 whys

I have a theory that the truth is never told during the nine-to-five hours.
Hunter S. Thompson

The 5 whys method is one of my favorite methods because it is so simple to remember and almost naturally to apply in many situations. Whenever you are presented with a fact (about a task to implement, a bug, or a problem) use this method to uncover the true origin of this fact.

Given a problem, you ask "Why ... did this happen?", the answer you will find or be given is not to be taken at face value at first. Instead this cause should be further analyzed. This process is repeated at least five times, only then can you be sure to be on track of finding the true cause for the problem.

Of course, the number 5, is arbitrarily picked, and can be any number. The point is to be aware that humans have a bias for making assumptions and giving answers that follow these assumptions and thereby follow a false path of conclusion. This ability for quick decision making, using heuristics, has served humans for a long time, because it yields a higher chance of survival. Daniel Kahneman calls this System 1 in his book Thinking, Fast and Slow.

The 5 whys method is a deliberate process to break out of this simplified thought process and engage in true problem analysis (using System 2 thinking).

It can be visualized using this flow chart:

The root cause can be reached immediately after the first question, or require more than 5 iterations. What's needed is the persistence to ask the right questions, and here a great helper is to identify omissions and assumptions that were made when identifying the cause.

Assumptions are rather straightforward to identify, they are usually known and can be uncovered through discussing which assumptions were made when identifying the cause.

  • Problem: The car does not start
  • Why did this happen: The starter does not work
  • Cause: The battery is dead

The assumption in this analysis is that the dead battery does not supply power to the starter.

Omissions are harder to identify, but they are relevant when applying this technique in a discussion with another person. I especially love this part, and I use it regularly in interviews, or when discussing problems with colleagues or partners. Here you need to get creative to uncover information that was not given when providing a cause. The important part here is to keep it a discussion on eye-level, a problem-focused discovery, not a way to shift responsibility away from you. You yourself are responsible for helping to uncover the root cause of the problem. The 5 why method therefore is also a tool for self-reflection and will help to discover your own assumptions, omissions and contributions to a problem's root cause.

The 5 whys method is so powerful because of its simplicity ... but be careful not to come across as annoying when applying it. Showing persistence in uncovering fundamental truths takes practice, but always delivers.


This post is part of the 42 first principles series.