Five Whys Plus

Overview

Five Whys Plus is a post-localization root-cause analysis skill. It assumes a fault has already been localized to a specific component or subsystem (by thinking-scientific-method or equivalent hypothesis-differential debugging). From that starting point — the proximate cause is known — it chains "why" questions backward to reach the systemic root cause, with explicit guards against the technique's known failure modes.

Core Principle: Never ask "why" without evidence for the answer. Chain backward from localized fault to actionable root cause, and stop only when the cause passes a counterfactual test: "Would the problem NOT have occurred if this cause were absent?"

When to Use

• A fault has been localized to a specific component (use thinking-scientific-method first if you don't know which component is at fault)
• The proximate cause is known but the systemic cause isn't
• A problem keeps recurring despite previous fixes
• You need the root cause, not just the proximate one

Decision flow: