5why-root-cause

5-Why Root Cause Analysis

Goal

Identify and validate a systemic root cause through a structured, evidence-based Why chain that can be directly linked to a corrective action.

Required Execution Checklist

• Problem defined with measurable data (value vs. specification)
• Linear Why chain built — one cause path at a time, no branching
• Each Why supported by objective evidence (not opinion or "probably")
• Logical consistency verified — reversal check passed
• Systemic root cause identified (not "human error" or symptom)
• Root cause validated: reproducibility test + elimination test
• Output enables definition of corrective action (PCA or CAPA)

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When to use

Use 5-Why for any quality problem with a single, linear cause chain. Works best for process failures, equipment failures, and procedural non-compliances. For complex problems with multiple contributing causes, use Fishbone first to identify candidate root causes, then use 5-Why to drill into each.

If multiple potential cause paths exist — do not combine them into one chain. Run separate 5-Why chains for each path and determine which is confirmed by evidence.

Prerequisites

• Problem is clearly defined (use Is/Is-Not to scope if needed)
• The investigation team has access to the process, machine, or system where the defect occurred
• Physical evidence or production records are available

Use within 8D — Occurrence and Escape

When used for 8D D4, two separate Why chains are required:

Chain	Starting question	What it finds
Occurrence chain	"Why did the defect happen?"	Root Cause of Occurrence (RCO) — the process or design failure that created the defect
Escape chain	"Why was the defect not detected before reaching the customer?"	Root Cause of Escape (RCE) — the detection system failure

Run each chain independently. Do not mix occurrence and escape causes in the same chain — they have different corrective actions. The occurrence chain addresses prevention; the escape chain addresses detection.