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:

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.

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Workflow

Step 1 — State the symptom precisely

Start with a factual, measurable problem statement. Not "the part was bad" but "dimension X measured 23.5mm; specification is 25.0 ± 0.5mm on part number Y, batch 2026-05-12."

The first Why answers: "Why did [specific symptom] occur?"

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Step 2 — Build the Why chain

For each Why, follow these rules:

Rule 1: Each answer must be supported by evidence

• Acceptable evidence: measurement data, production records, machine logs, direct observation, physical samples
• Not acceptable: "probably," "maybe," "someone said"

Rule 2: Each answer must logically lead to the next Why

• Test reversibility: "Because [Why N], therefore [Why N+1]"
• If the sentence doesn't make logical sense, the chain is broken

Rule 3: Keep the chain focused

• One Why chain = one cause path
• If you reach a fork (two possible causes), investigate both separately

Rule 4: Stop at a systemic root cause

• You have reached root cause when: fixing it would prevent the defect from ever recurring
• Typical systemic root causes: missing procedure, inadequate training system, process design gap, absence of poka-yoke, measurement system failure

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Step 3 — Validate the chain

Work backwards through the completed chain:

"Because [Why 5], therefore [Why 4]. Because [Why 4], therefore [Why 3]..."

If any step does not logically follow, revise that step.

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Step 4 — Validate the root cause

The root cause is correct if:

• Reproducibility test: can you deliberately create the defect by triggering the root cause? Yes → validated
• Elimination test: if you fix the root cause, does the defect disappear? (trial run or logical test)

If neither test is possible, the root cause is not confirmed — it is a hypothesis. Do not close the investigation on a hypothesis. Either obtain evidence to confirm it, or investigate alternative cause paths.

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