five-whys

Five Whys

Overview

The Five Whys is a root-cause analysis technique developed by Taiichi Ohno at Toyota. You start with a visible symptom and ask "Why did this happen?" repeatedly — each answer becomes the subject of the next question — until you reach the systemic cause rather than stopping at the proximate one.

Problem (symptom)
  └─ Why? → Cause 1
       └─ Why? → Cause 2
            └─ Why? → Cause 3
                 └─ Why? → Cause 4
                      └─ Why? → ROOT CAUSE (act here)

Five is a guideline, not a rule. Stop when you reach a cause that is actionable and where fixing it would prevent recurrence.

Core Concepts

Symptom vs. Root Cause

The symptom is what you observe; the root cause is the condition that made the symptom inevitable. Fixing a symptom stops this instance; fixing the root cause prevents the class of problem.

The Causal Chain

Each why step must be a direct cause of the step above it — not a parallel factor, not a contributing influence. If you can ask "but also why?" and get a different chain, you may have a branching problem requiring a fishbone diagram instead.

Actionability

A valid root cause must be something a person or team can actually change. "The system is complex" is not actionable. "No one owns monitoring for this service" is.

The "Five" Number

Ohno chose five because most surface-level explanations exhaust themselves in 2–3 whys; the 4th and 5th force you past the obvious. In practice, chains run 3–7 steps depending on domain.

How to Apply

Step 1 — Write a precise problem statement

Do not start with vague language. Bad: "the deployment broke." Good: "The production deployment at 14:32 UTC caused a 23-minute outage for all EU users." Ambiguous problem statements produce ambiguous root causes.

Step 2 — Ask the first "Why?"

What directly caused the problem statement to be true? Write it down. This should be a fact, not a guess — verify with data, logs, or direct observation if needed.